Are you an advanced GP user who would like to create your own customized version of GP? Are you comfortable working with textual code and using GIT -- or willing to learn those skills?
Now, thanks to MSandro, you can create fork your own GP mod on Github by forking this repository:
https://github.com/jhmaloney/GP-Mods
The process is pretty easy. You'll need to create a (free) Github account if you don't have one. Login, then go to above repository and click the "Fork" button in the top-right (just below the login/profile button). That will create your own fork on github. You can then "clone" that repository on your own computer and start modding!
There is a new category in the forums called "GP Mods" for topics related to modding GP. The old "Advanced Topics" category has been merged into "GP Mods" with some minor reorganizations (e.g. the separate Tips and Tutorials forums were merged).
If you create a mod, please tell us about it and share a link to your Github repository in the "Share your mod" forum.
Happy GP modding!