Since Gliscor was introduced in Gen IV, the only two moves that Gligar can learn that Gliscor can't through level up are Poison Sting and Slash. Poison Sting is learnt at level 1 however, and shouldn't ever be used as a substitute to any other Physical move, so Slash is the only move I'll care to focus on. It's worth noting that in every game these two Pokemon are accessible, Gliscor learns Night Slash at the same level Gligar learns Slash (level 31 in Gen IV, 34 in BW, and 27 in B2W2 and all later games), and Gligar and Gliscor's level up movepool are otherwise identical.
In Gen IV, although Slash certainly has a niche use in granting Gliscor neutral coverage, much of its moveset is reliant on TMs, as it learns no STAB moves through level-up. Chances are, you're going to teach Gligar a move like Aerial Ace or Dig through TM before you're even able to evolve it, so there's no reason to prolong its evolution to learn Slash. Even if you don't decide to teach it any STAB moves through TMs, moves like Strength and Return are stronger Normal type moves that are typically learnt before reaching that.
In Gen V and beyond, Gligar learns Acrobatics before Slash, a powerful STAB move that, combined with Night Slash, has neutral coverage on everything but a handful of Pokemon you aren't likely to ever encounter, of which are weak to Ground moves anyways. You therefore have no real reason to learn Slash or prolong Gligar's evolution.
In Gen IX, there is no cost to relearning moves, so Gliscor can learn Poison Jab, Fire Fang, Thunder Fang, and Ice Fang upon evolution. As a result, I'd recommend evolving Gligar as soon as possible for it to be able to learn these moves.
