What's the problem? 336 are easy to work on and the loading gate is just a piece of spring steel. Most problems I have seen with gates are the screw that hold it comes loose and they slightly get ajar and bind a bit. Otherwise the mag tube if it is insertion issues. Not uncommon for the cap to get sideways on the spring.
That's the carrier which can sometimes be rough. The screw that holds it is near the crossbow safety on the right side. If no safety it is basically the one directly below the hammer. Alot of people mistakenly use the loading gate screw. Has this gun every been disassembled for full cleaning? It is a very easy gun to work on and this is something you should learn if you don't know how to already. The lug rides the carrier which when the loop is fully seated is what allows it to be fired. The lug pushes a 2 part firing pin up and also helps retain the carrier.
And I've dealt with Jeff before he got that new location and he is pretty good general gunsmith if your looking for a someone with a business.
I just know this gun is easy to work on and am sure you could do it yourself and save the money.