I had the same problem with my GT2. First of all, make sure the engine isn't flooded, tip it upside down and pull the starter, if fuel comes out that might have been your problem. Second, have you been able to move the piston a little or is it just "stuck"? If it's stuck your engine may be damaged, either that or you have a serious flooding, or you could be stuck at TDC (top dead center). What I did was try to clear the inside of the engine of as much fuel as possible and just keep trying to pull it, and if it didn't pull right I'd loosen the glow plug, retighten it and tried again. I eventually got it started.