GMC: the jsonstring escape need to be improved

[KfreeZ]

For security concern, #243 introduced the escapes of jsonstring to avoid code scan alert.
However, it's a quick fix, the more detailed improvement need to be done and a full UT is required.
Thanks @mkbhanda point this out and this issue is to track the following up.
the json spec:
https://www.json.org/json-en.html