Will I be allowed to make fork of GD? It will have same license as GD (GPL+LGPL).
Reason I’m asking is that even though few ideas I have for GD are compatible with general concept and those will be offered as patches, other aren’t such as component based design (think Stencyl’s behaviors) and some will break backward compatibility (I can’t release details of this one yet because I’m still figuring out how I’ll implement this idea).
As long as the licenses are respected, it should be ok but try not to do this.
Even if you can make fork to do some experiments, try to stick with the master branch of GD because nothing good will happen if everyone start working on their own version of GD: this is something I do not want to happen.
I make GD open source so that everyone can contribute to the software, not to have 15 useless forks of it.
Yeah, some features I’ll definitely commit to main repo, but some are going to be outside of your scope and your design ideas and those are more likely to be applicable for a fork.
//edit: Of course if you believe you’d like some of features in fork even if they’re different than main gd, I’ll be more than happy to provide diffs.
Yeah sounds good, and GitHub will make the merge of potential new features easy thanks to the pull request mechanism. 
Yep, but I’m a bit of a git noob - All I can do is commit and even that only with tools like TortoiseGit.
With GitHub it’s quite easy: you go on the GD repository, you click on “Fork” to copy the repository, you made some changes, you commit and if you want me to merge these changes with the official GD repository, you click on “Pull request”.
In any case, I’m still investigating how to ensure that developers will be able to work on the extensions without having to compile the whole IDE and platforms code. Surely with some options to (un)check during in CMake.