If you are looking for info, try calling Coyote Creek in Rochester NH. One of the older guys or owner would have information.
The Contender muzzleloader barrels were some of the early work for Gonic Arms. The Story I hear was that the owner worked as a gunsmith at TC and suggested to TC to make a muzzleloader contender. TC was not interested early on and he patented the muzzleloader design design for the G1 contender. TC later on did may the .45 muzzleloader for the G2 only and was setup to not close the action on the G1. This avoided TC being sued by Gonic.
Gonic Arms is out of business. The owners son ran it for a short time but had no skill at running the operation. Good luck getting parts, some of the tooling is for sale on ebay currently. If the Breech plug is in good shape you could try and install a ventliner. I would not pay much for the barrel no mater the condition due to limited parts. I would also slug the bore to correctly size your bullets. Gonic arms early on made really accurate barrels and rifles. The stuff is son made was not consistently high quality.
Did you ever sell the TC contender .50 cal barrel? If not consider putting a ventliner in the breech plug.
You are better informed than most. Fred Rodney, founder of Gonic Arms, did indeed work for TC.
Gonic Arms was founded by Fred Rodney a former employee of Thompson Center. He came out with a line of inline muzzle-loaders and custom muzzle loading contender barrel in lengths from 16 to 24 inches. Unfortunately Fred Rodney died and eventually the business went with him.
They were known for some fabulous performance guns such as the GA-87 (below) and the GA-93 also the GA-90 for contenders.
I've got a GA-93 SS and two GA-90's in 45 cal (one 16" and one 20")
The GA-90's use 209 primers and the GA-93 use a #11 cap. They all are excellent shooters