Is there a way to use Gamemaker with Gource? Or is there something like a tool or library to export / convert .gmk files to .txt files each of them with one class?
While pre-Studio projects are stored as single files, you can use Gmk-Splitter to split the .gmk file into a number of XML files (not too unlike the format used by GameMaker: Studio) or recompose the file from a set.
While not ideal, this permits to use source control with older versions of GameMaker. See the author's "Gang-Garrison-2" repository for an example of that in action.