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