Is your GUMpaste storebought or homemade?
Homemade paste tends to be a bit stiff if you're not careful when making it. Stiff paste dries faster so if you don't work quickly enough, stuff'll crack. Could also be that things aren't thin enough. Strange but true.
Do the projects crack while they're being made or while/after drying?
If you don't knead throughly so that the paste is SMOOTH, cracking will happen as it dries.
How long have you been...