As gawk is Free Software, the source code is always available. Gawk Distribution, describes how to get and build the formal, released versions of gawk.
However, if you want to modify gawk and contribute back your changes, you will probably wish to work with the development version. To do so, you will need to access the gawk source code repository. The code is maintained using the Git distributed version control system. You will need to install it if your system doesn't have it. Once you have done so, use the command:
git clone git://git.savannah.gnu.org/gawk.git
This will clone the gawk repository. If you are behind a firewall that will not allow you to use the Git native protocol, you can still access the repository using:
git clone http://git.savannah.gnu.org/r/gawk.git
Once you have made changes, you can use ‘git diff’ to produce a patch, and send that to the gawk maintainer; see Bugs, for how to do that.
Once upon a time there was Git–CVS gateway for use by people who could not install Git. However, this gateway no longer works, so you may have better luck using a more modern version control system like Bazaar, that has a Git plug-in for working with Git repositories.