What is the GCWR for a kenworth t800?

Any class 8 truck can do 80k. The GWR you are looking at is just for the power unit. The towing capacity is generally only limited by what the truck is registered for. 80k for interstate and (around here) 105k for tri axle trailers. You have typically two 20k drive axles, but that tends to be limited to 34k by STAA laws. That leaves you 14k left for the from axle, which tends to land somewhere between 12k and 12.5k lbs.
The GCWR really doesn't matter as the rest of the weight falls on the trailer axles.
The frame, engine, transmission, and rear end call all handle up to 105k just as easy as 80k. The only difference I've seen at the companies that have both is the cab registration and the tri axles have a dump switch for the third axle.
Heck, I've seen a fair few times they pulled one of the 80k trucks and simply had the shop wire a dump switch to have an extra truck just in case.

 

What didn't you like about pulling it?

I've never pulled a pintle hitch trailer, but as long as it stops good & is stable, I don't see the issue.

Like @ZVar said, the truck itself is surely capable.

 

48,000 gvw is probably 38 or even 36 rears depending how old it is, total would depend on what the trailer is good for

 

Not so fast, even what I told you is simply "general" information.
On your door jamb is the axle weights on a per axle basis.
If it's a 12k or better axle, then yea you might want to get the suspension looked at. It could be specced smaller to save money for a specific task.

 

Something is wrong with your truck.

I’m at 80k all the time in a dump truck pulling a pintle hitch. The truck actually rides better with a load and heavy trailer.