I have a ‘77 competition (58cm), which was the first year for the ‘GS’, and the tire clearance was actually reduced, with the changed frame. I can fit 35’s but it’s especially tight in the rear and fenders are a no-go. I could run larger if I ‘crimped’ the tubing on the inside. I run 32’s (panaracer gravelking’s) on a set of h+Son tb14’s and they are more than good enough for a typical hardpack gravel road. Fenders are still a no-go though. I would need to go down to a 28, to have the clearance for fenders, particularly in the rear.

unless you’re an especially heavy rider, or you’re dealing with proper singletrack, not a general unpaved road, I would think a 32 or a 35 if you prefer, would be all you would need.

as far as your crankset, your ring choices are extremely limited. I believe the TA had the same 116bcd as the campagnolo gransport crank. There were a few other makes that used this design, including simplex and nervar. the inner ring was bolted to the outer ring on the campagnolo version, so I’m not sure if those inner rings would work on yours, but the campy version did have smaller 38-39 inner rings. eBay is likelybyour best bet, but I don’t think it’s worth it. My advice would be to sell the crankset and update it. Also, that long cage jubilee derailleur is pretty rare. You could get a pretty penny for that. If you want to maintain some authenticity, you could switch to a vintage TA 5-pin (50.4bcd) ‘cyclotouriste’ crankset and get some new TA ‘pro-5 vis’ or velo-orange rings. That’s probably what I would do.