Well..
Not exactly - the GMARKER isn't illegal, but the firing mode is. So if you have a marker that has a burst mode, you have to lock it out so you can't get into 3-burst mode without tools if you plan on playing any tournaments.
Although there are a few people (myself included) considering banning any marker with enhanced firing modes all together to discourage their manufacture, whether you can lock out the mode or not.
- Chris