I've accidentally found an Obfuscator for GM yesterday.
http://gmc.yoyogames.com/index.php?showtopic=373402I know it works with GM 5, 6, 7, & 8.(I haven't tested GM 8.1 yet.)
Note: It doesn't make it undecompilable. So it's best to use an anti-decompiler with it.Note: This doesn't need to be used, it's only here for those who wish to use it.