It does not matter if you found a soundness issue in typechecker or found the documentation confusing. Either way filing a an issue to the issue tracker is extremely helpful.
A good place to start is to look at the issues marked as beginner which are issues that should be possible to work on without knowledge on the inner workings of Gluon.
If you find something that looks interesting then please leave a comment on the issue, that way you can get assistance quicker and there is no risk of duplicating work.
Gluon can build with version 1.9.0 of Rust or later but we recommend to use version 1.11.0 or later to avoid some very long compile times for the gluon_parser crate.
To build and run all tests for Gluon you can use the test.sh script. If you look inside it you can see that it actually builds from the c-api directory (this is necessary as the gluon_c-api crate depends on the gluon crate). This means that all build artifacts end up in c-api/target. For this reason if you want to run tests only for a single crate you should run something like (cd c-api && cargo test --features test -p gluon_check testname).
If you are building with a nightly version of rust which supports workspaces you can use the test-nightly.sh script instead which puts the build artifacts in the root folder instead of in c-api.
Once you have made some changes you will need to file a pull request to get your changes merged into the main repository. If the code is still a work in progress it can still be a good idea to submit a PR as that will let other contributors see your progress and provide assistance (you may prefix the PR message with [WIP] to make it explicit that the PR is incomplete).
You may see that some of the commits follow the commit message convention of Angular. Following this convention is optional but if you enjoy using it then feel free to do so!