From main site :Why is Gligar not banned ? I thought Little Cup rules stated that the Pokemon competing couldn't be ever at any point in time be a only evolution. Which he was in GSC.
Gligar is considered to be one of the most dangerous Pokemon in the current Little Cup metagame for a few reasons: a solid offensive and defensive typing, a wide movepool that allows sets to be easily be customized, and the ability to reach 19 Speed. He is also bulky enough to take some hits and has the ability to run a plethora of different sets. So why isn't he banned from the standard Little Cup metagame? Firstly, he is weak to Ice- and Water-type attacks, which are two of the most common attacking types in Little Cup. Additionally, each type features a strong priority attack in Ice Shard and Aqua Jet. He also has quite a few checks and counters. That isn't to say he can't play around them, but they make it difficult for him to sweep teams effectively. Once said Pokemon are gone, however, it is easy for Gligar to rip teams apart.