first off i would check the voltage at the starter solenoid "start wire" from the keyswitch.
it should be battery volts, ANY voltage drop can cause a solenoid engage failure, it should have enough to pull it in correctly. Check voltage AT THE BATTERY, then check the voltage at the "start wire" on the solenoid while turning the keyswitch to the start position. If you see a voltage drop then you will need to back trace and find the resistance point.
You could have a bad keyswitch, or possibly a bad neutral start switch as well.
This is always the most common issue with no start problems when the starter will not engage (after the battery and starter have been checked)

Does the GP30 have the long lever on the left of the steering column for a fwd/rev shifter?
If so then there is a switch down at the base of that shifter, once it gets age on it somethings they do not want to stay in neutral and can cause the truck not to start until you move it back to neutral. As far as the GP30K it has the shifter up on the column and the neutral start position generally isn't an issue unless the switch is bad.

Note: having both serial numbers of both units would help alot to be able to get the right information for your units.