Shimano 100GS group (xx-M100) was made 1991-2. I know of no on-line docs for the group.
1) Wash out/soak the shifter with something like WD-40 until it works, then spray in a light lube if you have one. Exact procedures vary.
2) It's probably a matter of adjustment of the FD, but giving the left shifter the same treatment couldn't hurt.
Also check that the cables are smooth in the housings.
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