If you would like to help pretest GCC releases to assure they work well, current development sources are available via Git (see https://gcc.gnu.org/git.html). Source and binary snapshots are also available for FTP; see https://gcc.gnu.org/snapshots.html.
If you would like to work on improvements to GCC, please read the advice at these URLs:
for information on how to make useful contributions and avoid duplication of effort. Suggested projects are listed at https://gcc.gnu.org/projects/.