The source code for Gnumeric lives in Gnome's git system. The standard directions can be used to check out and build the necessary modules. You will definitely need the 'gnumeric' module. The development series frequently requires unreleased versions of libgoffice (module goffice) or libgsf (module libgsf).
If you wish to submit patches, the best way is to open a new bug report and attach the patch.
We strive to test every sheet function in Gnumeric, preferably automatically in our test suite (which uses files generally stored in the our samples collection). We do not have complete coverage yet — help and pointers to certified test values are most welcome.
As always, patience and politeness will help in getting assistance. Respect the users and developers, and they will respect you.