Ducky,
The braking power of your truck is based on its gvwr. It has NOTHING to do with gcwr. GCWR is strictly a get up and go/performance power factor! Braking has to do with the tow rigs brakes, along with the towed rigs brakes. BOTH sets of brakes need to work in conjunction to stop a rig in the alloted time you have per the state/jurisdiction you are traveling in. Manufactures GCWR has NOTHING to do with those laws! In fact, most states do not recognize manufactures gcwr numbers, many like the one I live in, do not follow, nor recognize the manufacture numbers from a what I can go down the road at in a legal fashion gvwr wise! I've even been pulled over commercially WELL over the manufactures ratings, and have yet to get an over wt ticket.
Really it is up to the operator to use multiple factors in figuring out if the tow rig etc is safe enough for that drivers needs!
Marty