obligatory battlebots 5 comment
You got my vote for RA2 Wizard. Always and forever.
All the decompiler does is make everything more readable (after starting up the decompiler, select the compiled gmf and click 'Open'; then pick a new name for the decompiled version and click 'Save'). You still need to know what your doing in order for your changes to work (after recompiling). Take a look at the Beetlebro's tutorial (on their site) for a slightly-overwhelming, beginner's look at the GMF. Take a look at other GMF files for the more advanced stuff (such as how to make arenas).