What model receiver is on the roof? Sounds like you have a JD receiver plugged into the greenfit. You should easily get GPS speed for the planter as long as the harnesses all check out. 

I'd be testing and checking GPS status and such without the planter hooked up. Then test the same stuff with planter hooked up. Depending on what seedstar version the planter has, you will have speed source selection on different pages. And be able to see if it's reading or not while you're moving 

But I agree, if you moved the G5 and receiver to an R series JD tractor and it didn't work, then you have a display or GPS receiver issue. Software or bent pin or something.