can you post a full model number?GDP120??? <-- what comes after the 120?
on a general note, the shifters on these machines have a spring loaded detent plate in the shifter mechanism on the steering column, if those detent plates or the spring loaded pins wear out or the adjustment becomes loose it could allow it to pop out of place because of vibration while driving. Being you are in forward the shifter is in the upward position and gravity may allow it to drop out back to neutral due to gravity and vibration. There should be a tension adjuster on the detent mechanism's to increase the spring pressure. But make sure the tracks these detents ride on are not worn out. Usually they get hard to shift because of lack of lubrication and a shot of lube fixes that issue but in your case it sounds like they are worn out in the forward position.
You'll have to remove the steering column cover to access that area where the shifter is to examine it and see if this is the case. Worse case scenario is that the shifter is just worn out and will have to be replaced.