The source code of VVVVVV is now public! https://github.com/TerryCavanagh/vvvvvv
youre doing it wrongand we cant help if you dont show us what you did
flag(1,on)
customifflag(1,script3)
destroy(gravitylines)
customifflag is an internal command to load a script, try ifflag instead.
Actually, ifflag is an internal command as well, it just loads scripts from the main game, so it's mostly useless compared to customifflag.