It does not matter if you found a soundness issue in typechecker or found the documentation confusing. Either way filing an issue to the issue tracker is extremely helpful.
A good place to start is to look at the issues marked as beginner. These are issues that should be possible to work on without knowledge on the inner workings of Gluon.
If you find something that looks interesting, 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 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 call cargo test --features test --all. Instead of --all you can pass the -p <crate name> and --test <test module> flags to compile a specific crate and/or test module. For instance, cargo test --features test -p gluon_parser --test basic to run the tests in parsers/tests/basic.rs.
(*) You can see what Travis CI actually builds and tests in scripts/travis.sh. Most of the time you should not need to worry about these additional tests and can just rely on travis running them.
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. 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, feel free to do so!