diff --git a/modules/gptPreAuction.js b/modules/gptPreAuction.js
index 2dc5a6983b9..88f90419aeb 100644
--- a/modules/gptPreAuction.js
+++ b/modules/gptPreAuction.js
@@ -48,10 +48,35 @@ const sanitizeSlotPath = (path) => {
   return path;
 }
 
+const defaultPreAuction = (adUnit, adServerAdSlot) => {
+  const context = adUnit.ortb2Imp.ext.data;
+
+  // use pbadslot if supplied
+  if (context.pbadslot) {
+    return context.pbadslot;
+  }
+
+  // confirm that GPT is set up
+  if (!isGptPubadsDefined()) {
+    return;
+  }
+
+  // find all GPT slots with this name
+  var gptSlots = window.googletag.pubads().getSlots().filter(slot => slot.getAdUnitPath() === adServerAdSlot);
+
+  if (gptSlots.length === 0) {
+    return; // should never happen
+  }
+
+  if (gptSlots.length === 1) {
+    return adServerAdSlot;
+  }
+
+  // else the adunit code must be div id. append it.
+  return `${adServerAdSlot}#${adUnit.code}`;
+}
+
 export const appendPbAdSlot = adUnit => {
-  adUnit.ortb2Imp = adUnit.ortb2Imp || {};
-  adUnit.ortb2Imp.ext = adUnit.ortb2Imp.ext || {};
-  adUnit.ortb2Imp.ext.data = adUnit.ortb2Imp.ext.data || {};
   const context = adUnit.ortb2Imp.ext.data;
   const { customPbAdSlot } = _currentConfig;
 
@@ -84,11 +109,32 @@ export const appendPbAdSlot = adUnit => {
 
 export const makeBidRequestsHook = (fn, adUnits, ...args) => {
   appendGptSlots(adUnits);
+  const { useDefaultPreAuction, customPreAuction } = _currentConfig;
   adUnits.forEach(adUnit => {
-    const usedAdUnitCode = appendPbAdSlot(adUnit);
-    // gpid should be set to itself if already set, or to what pbadslot was (as long as it was not adUnit code)
-    if (!adUnit.ortb2Imp.ext.gpid && !usedAdUnitCode) {
-      adUnit.ortb2Imp.ext.gpid = adUnit.ortb2Imp.ext.data.pbadslot;
+    // init the ortb2Imp if not done yet
+    adUnit.ortb2Imp = adUnit.ortb2Imp || {};
+    adUnit.ortb2Imp.ext = adUnit.ortb2Imp.ext || {};
+    adUnit.ortb2Imp.ext.data = adUnit.ortb2Imp.ext.data || {};
+    const context = adUnit.ortb2Imp.ext;
+
+    // if neither new confs set do old stuff
+    if (!customPreAuction && !useDefaultPreAuction) {
+      const usedAdUnitCode = appendPbAdSlot(adUnit);
+      // gpid should be set to itself if already set, or to what pbadslot was (as long as it was not adUnit code)
+      if (!context.gpid && !usedAdUnitCode) {
+        context.gpid = context.data.pbadslot;
+      }
+    } else {
+      let adserverSlot = deepAccess(context, 'data.adserver.adslot');
+      let result;
+      if (customPreAuction) {
+        result = customPreAuction(adUnit, adserverSlot);
+      } else if (useDefaultPreAuction) {
+        result = defaultPreAuction(adUnit, adserverSlot);
+      }
+      if (result) {
+        context.gpid = context.data.pbadslot = result;
+      }
     }
   });
   return fn.call(this, adUnits, ...args);
@@ -100,6 +146,8 @@ const handleSetGptConfig = moduleConfig => {
     'customGptSlotMatching', customGptSlotMatching =>
       typeof customGptSlotMatching === 'function' && customGptSlotMatching,
     'customPbAdSlot', customPbAdSlot => typeof customPbAdSlot === 'function' && customPbAdSlot,
+    'customPreAuction', customPreAuction => typeof customPreAuction === 'function' && customPreAuction,
+    'useDefaultPreAuction', useDefaultPreAuction => useDefaultPreAuction === true,
   ]);
 
   if (_currentConfig.enabled) {
diff --git a/test/spec/modules/gptPreAuction_spec.js b/test/spec/modules/gptPreAuction_spec.js
index 9e81aea80d1..2bfce71806e 100644
--- a/test/spec/modules/gptPreAuction_spec.js
+++ b/test/spec/modules/gptPreAuction_spec.js
@@ -20,7 +20,9 @@ describe('GPT pre-auction module', () => {
   const testSlots = [
     makeSlot({ code: 'slotCode1', divId: 'div1' }),
     makeSlot({ code: 'slotCode2', divId: 'div2' }),
-    makeSlot({ code: 'slotCode3', divId: 'div3' })
+    makeSlot({ code: 'slotCode3', divId: 'div3' }),
+    makeSlot({ code: 'slotCode4', divId: 'div4' }),
+    makeSlot({ code: 'slotCode4', divId: 'div5' })
   ];
 
   describe('appendPbAdSlot', () => {
@@ -172,7 +174,9 @@ describe('GPT pre-auction module', () => {
       expect(_currentConfig).to.deep.equal({
         enabled: true,
         customGptSlotMatching: false,
-        customPbAdSlot: false
+        customPbAdSlot: false,
+        customPreAuction: false,
+        useDefaultPreAuction: false
       });
     });
   });
@@ -266,5 +270,176 @@ describe('GPT pre-auction module', () => {
       runMakeBidRequests(testAdUnits);
       expect(returnedAdUnits).to.deep.equal(expectedAdUnits);
     });
+
+    it('should use the passed customPreAuction logic', () => {
+      let counter = 0;
+      config.setConfig({
+        gptPreAuction: {
+          enabled: true,
+          customPreAuction: (adUnit, slotName) => {
+            counter += 1;
+            return `${adUnit.code}-${slotName || counter}`;
+          }
+        }
+      });
+      const testAdUnits = [
+        {
+          code: 'adUnit1',
+          ortb2Imp: { ext: { data: { pbadslot: '12345' } } }
+        },
+        {
+          code: 'adUnit2',
+        },
+        {
+          code: 'slotCode3',
+        },
+        {
+          code: 'div4',
+        }
+      ];
+
+      // all slots should be passed in same time and have slot-${index}
+      const expectedAdUnits = [{
+        code: 'adUnit1',
+        ortb2Imp: {
+          ext: {
+            // no slotname match so uses adUnit.code-counter
+            data: {
+              pbadslot: 'adUnit1-1'
+            },
+            gpid: 'adUnit1-1'
+          }
+        }
+      },
+      // second adunit
+      {
+        code: 'adUnit2',
+        ortb2Imp: {
+          ext: {
+            // no slotname match so uses adUnit.code-counter
+            data: {
+              pbadslot: 'adUnit2-2'
+            },
+            gpid: 'adUnit2-2'
+          }
+        }
+      }, {
+        code: 'slotCode3',
+        ortb2Imp: {
+          ext: {
+            // slotname found, so uses code + slotname (which is same)
+            data: {
+              pbadslot: 'slotCode3-slotCode3',
+              adserver: {
+                name: 'gam',
+                adslot: 'slotCode3'
+              }
+            },
+            gpid: 'slotCode3-slotCode3'
+          }
+        }
+      }, {
+        code: 'div4',
+        ortb2Imp: {
+          ext: {
+            // slotname found, so uses code + slotname
+            data: {
+              pbadslot: 'div4-slotCode4',
+              adserver: {
+                name: 'gam',
+                adslot: 'slotCode4'
+              }
+            },
+            gpid: 'div4-slotCode4'
+          }
+        }
+      }];
+
+      window.googletag.pubads().setSlots(testSlots);
+      runMakeBidRequests(testAdUnits);
+      expect(returnedAdUnits).to.deep.equal(expectedAdUnits);
+    });
+
+    it('should use useDefaultPreAuction logic', () => {
+      config.setConfig({
+        gptPreAuction: {
+          enabled: true,
+          useDefaultPreAuction: true
+        }
+      });
+      const testAdUnits = [
+        // First adUnit should use the preset pbadslot
+        {
+          code: 'adUnit1',
+          ortb2Imp: { ext: { data: { pbadslot: '12345' } } }
+        },
+        // Second adUnit should not match a gam slot, so no slot set
+        {
+          code: 'adUnit2',
+        },
+        // third adunit matches a single slot so uses it
+        {
+          code: 'slotCode3',
+        },
+        // fourth adunit matches multiple slots so combination
+        {
+          code: 'div4',
+        }
+      ];
+
+      const expectedAdUnits = [{
+        code: 'adUnit1',
+        ortb2Imp: {
+          ext: {
+            data: {
+              pbadslot: '12345'
+            },
+            gpid: '12345'
+          }
+        }
+      },
+      // second adunit
+      {
+        code: 'adUnit2',
+        ortb2Imp: {
+          ext: {
+            data: {
+            },
+          }
+        }
+      }, {
+        code: 'slotCode3',
+        ortb2Imp: {
+          ext: {
+            data: {
+              pbadslot: 'slotCode3',
+              adserver: {
+                name: 'gam',
+                adslot: 'slotCode3'
+              }
+            },
+            gpid: 'slotCode3'
+          }
+        }
+      }, {
+        code: 'div4',
+        ortb2Imp: {
+          ext: {
+            data: {
+              pbadslot: 'slotCode4#div4',
+              adserver: {
+                name: 'gam',
+                adslot: 'slotCode4'
+              }
+            },
+            gpid: 'slotCode4#div4'
+          }
+        }
+      }];
+
+      window.googletag.pubads().setSlots(testSlots);
+      runMakeBidRequests(testAdUnits);
+      expect(returnedAdUnits).to.deep.equal(expectedAdUnits);
+    });
   });
 });