I have done quite a bit of poking around for the gecko source but I cannot seem to find it.
I have found the word gecko littered through the firefox source code when I do this:
for i in `find .`; do grep -ibH gecko $i; done
If anyone knows where I can get the source I would really appretiate it.
Most of the code in mozilla-central is Gecko. Gecko isn't a precisely-defined term, though. The mozilla-central repository also contains code that's not Gecko, including Firefox (in browser/).
A good link for how to get the source is https://developer.mozilla.org/En/Developer_Guide/Source_Code/Mercurial