Yes, both 6 and 7-speed road hubs are 126mm. With most setups, you can go from 6 to 7 with no problems. But some frame/hub/freewheel combos may require adding a 1mm axle spacer to the drive side to give enough clearance. Sounds like this is what you have. If you add the spacer, you will have to tweak derailer stops and indexing barrel adjuster to get it all working again since the freewheel will be further away from the derailer.

Redishing by itself doesn't solve the problem. But if you put more than 1mm of spacers, you may want to redish. Personally, I wouldn't bother redishing if only 1mm is required (unless you take 1mm from the non-drive side and move it to the drive side).

Note: The Suntour 7-speed freewheels are just a tiny bit (0.6mm?) narrower than Shimano (Sachs probably uses the Shimano spacing) so the GPX stuff could have been 7-speed and still had enough clearance.