I haven't seen a GUI editor that can support Pawn's weird syntax.
However, Notepad++ sort of works under wine. A few people use Crimson Editor on Windows, maybe it too works under wine.
I use vim, and for syntax highlighting I just made vim think .sma/.inc/.sp files are C files, but it has some quirks, like the auto indent doesn't want to work. I've been meaning to look into creating a true Pawn vim config.