Personally, every aftermarket system I've installed has been poorly integrated and caused more trouble than it is worth. I like having it integrated into the vehicle so the messages display on the dash. OTOH, the Hummer TPMS is dumbed down so it provides minimal information.
A lot of people complain about the TPMS system, but mine has been reliable (although two sensors failed after I switched to the Hutchinson's, but I think that was an installation issue. Tustin Hummer, who installed the TPMS sensors in the Hutchinson beadlocks, replaced the two failed sensors and never charged me.)
What I don't know is how they disabled the TPMS system so it doesn't keep putting low tire pressure warnings on the dash. Probably a different software build to ignore the TPMS or lack thereof. Maybe ask Mike at Tustin Hummer if the system can be reactivated?
If you find a good deal on the TPMS sensors on ebay, you should be able to sell them the same way for what you paid less shipping if it doesn't work out.