Somebody here might have a firm answer for you, but it's going to be specific to your GMC truck, not to the trailer.
A typical add-on trailer TPMS has screw-on sensors for all the stems, and a repeater/booster unit mounted somewhere on the frontside to boost the signal to your truck receiver, due to the distance (as you note). You may or may not need the relay for a boat trailer, depending on signal strength and shorter distance. The signals are pretty strong -- my booster, located on the tongue next to my battery box, actually reads the sensor on my spare, mounted on the back bumper, "through" the entire length of my trailer.
But the big question is, what brand TPMS is your GMC's builtin receiver looking for? I would think the manual ought to cover that.
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2019 Cougar 26RBSWE
2019 Ford F-250
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