This very well could be. Proportional thermostats usually work fine with GFI's. I've only come across one or two cases where there was an issue. If I remember right the person replaced the GFI with a new one of the same model and then it was fine. So there may be some tolerance differences. If you weren't already I would use a surge protector with a 2000+ joule rating in between the thermostat and wall outlet. The protectors have a mov cap that I suspect works both ways and may keep the relay arc from tripping the GFI.