There's plenty of ways to fix this. Read this:
http://www.forum.simplify3d.com/viewtop ... 124&p=8613
You can just change the "set bed temperature" command in your firmware configuration to be "M140 S#" (just remove the T0 part). Done! Solved.
The developers are also aware of this, but unfortunately, it's actually an issue with the GPX convertor, not the Simplify3D code. From what they have told me, they are working to patch GPX to resolve it for everyone.