I can help you diagnose why the truck wont start, I just went through this process a few weeks ago.
1. Do you have pressure in the brake system? If you don't, truck won't start.
2. You can test the starter and starter circuit by jumping the relay, it will let you know if the problem is with the ECU or the starter system.
Frankly I wouldn't spend the $500 on a GAP tool before you try a normal OBDII tool... $15 tool from amazon or rent one from autozone.
Test the starter by removing the starter motor relay from the relay box (next to the battery in the engine bay). Under the cover, the relay is labeled "SM"... I want to say it is on the passenger side of the relay bank, close to the front. Pull the relay out.
The plug will look like the blue box on this diagram. Take a short piece of thick-ish wire, and connect pins 30 and 87. Make sure you jump the right pins, or else you run the risk of cooking your ECU: When you connect those two pins, the starter should engage.
No click? Starter bad or bad starter fuse.
Click but no crank? engine seized.
Engine cranks? ECU is not sending a start signal.