I have my projects on github(more for source control than for sharing though). If you drill down into the scripts folder of the project folder, you can find all your scripts, and the code itself is in .gml files. Github's web viewer is perfectly able to see them and show the code. It in fact even recognizes the fact that it IS gml and so you can filter it(it also things that all the .yy files are whatever the YACC language is). The viewer even has code letter coloring based on what it is(keyword, function, etc...). In fact, you can ALSO see code in object events, etc... as those are also stored in separate gml files. The only catch to those are that like named events will have numbers, like Step_2.gml being the End Step event for an object.
So yes, the code is certainly viewable on github. The catch is that there are plenty of other files that are not gml code so you have to either filter it or drill down into the directories to find what you need.