Automotive electronics professional (specializing in alarms and remote starters) checking in here.
To accomplish what you want, the relay can simply be used to interrupt the main ignition wire coming from the ignition key switch. As beowulff said, this would have pretty much the same effect as just turning the key off.
(You’d probably be interrupting only one ignition circuit with your relay , and the truck might have two or more ignition wires, as well as some accessory wires. Therefore, when the GPS unit energizes your relay, the engine would stop, but things like the radio and climate controls would likely remain operational until the ignition key is turned off completely.)
However, most people would recommend you interrupt the starter wire, not the ignition wire. Why?
If the ignition were to turn off with the vehicle in motion, the driver would lose engine power (of course), but also power steering and in short order, the power brakes too.
An astute, alert driver should theoretically be able to react and carefully coast to the side of the road, but it’s also possible to end up coming to a stop in the middle of a highway, or worse yet, losing control going around a curve.
This could happen not only to the car thief, but also to the legitimate owner in the event one of the wire connections comes loose, someone triggers the GPS unit by mistake, or (very rarely) the relay fails.
Therefore, you might want to consider a starter kill.
—If someone steals the truck, they’ll still be able to have safe control to their destination, until they shut it off. Then, they won’t be able to start again, and at this point the GPS unit can be used to locate the stolen truck.
—If a malfunction occurs while the legitimate truck owner is driving, the truck will still make it to its destination, with no roadside breakdowns. It just won’t start again until it’s fixed, that’s all.
—It’s much safer and easier to deal with a parked vehicle that won’t start, rather than a moving vehicle that shuts down.
Useful links for relay wiring: