You can evolve Gligar into Gliscor but not then into Gliscor Lv. X because Lv. X'ing a Pokemon doesn't count as evolving. If you wanted to, you could evolve Gligar to Gliscor, then use a Level Max and get heads to Lv. Up into Gliscor Lv. X. Although using that tactic, you won't be able to activate Gliscor Lv. X's Shoot Poison Poke-Power because you aren't playing it down from your hand.