Your best bet imo for deciding if something unevolved should be allowed in UU or not is following the following steps. We'll take Gligar as an example.
1) Completely ignore that it has an evolution. If Pokemon X has amazing stats, then evolves into Pokemon Y with lesser stats, which of the ones is more likely to be UU? I'd say Y. If Pokemon A has the same stats as Pokemon B and yet one evolves into the other, how different are they when actually tiering things? Whether they evolve and what they evolve in does not matter when you are already judging unevolved Pokemon already.
So we will look at Gligar, pretending Gamefreak wasn't kind enough to show his potential in the form of Gliscor. Now, we observe what we have...
2) Gligar was an UU Pokemon last generation. This does not serve as a direct argument for Gligar's tiering in DP due to all the drastic changes that have been done, and not just to Gligar but due to the whole UU border being changed around, mechanical changes and all, we can't really use Advance UU as an argument but it can be a starting point for estimating its power a bit.
3) Now, we look at Gligar's statistics. This is where you start spouting advantages and disadvantages. Gligar mostly has the former. Typing? Ground/Flying. Most notable things? 4x Ice weakness, resistance to Fighting while maintaining neutralty to Rock and being immune to Electric and Ground. Not weak to Stealth Rock and immune to Spikes and Toxic Spikes.
Stat-wise, Gligar has fairly average if not bad stats across the board except for the 105 Def...but overall workable enough, as expected from something that was used a lot in UU in Advance. 65/75/105/35/65/85 is not something you can simply say "wow way too fucking powerful" or "too goddamn sucky to make an impact" of, so we look a bit further.
Move-wise, Gligar has it covered. STABed Earthquake, arguably one of the best offensive aspects any Pokemon could muster. STAB Aerial Ace as well, for all it's worth. Then we have Swords Dance and elemental fangs, Night Slash, X-Scissor, Stone Edge and U-Turn to about round out Gligar's offensive movepool. Defensively, Gligar can tank with Roost, set up Stealth Rock, use Knock Off well and Baton Pass Swords Dances and Rock Carts. Taunt is there, too.
Now start comparing Gligar to how we can currently sketch the UU metagame. First off, if it sweeps about anything you can shake your finger at, you can bring up a case for not allowing it. But we have access to a lot of Waters that shut down Gligar pretty easily without going too far out of their usual ways - Poliwrath, Quagsire and Gastrodon come to mind. Of course, Gligar's main role was usually defensive.
Then you get to the most difficult part: is Gligar walling enough enemies, like Chansey does, to really make a strong case for banning it? It was always there to beat the hell out of Fighters and Ground types of all kinds...back in Advance you even had things like (the back then elemental) Ice Punch Muk/Primeape and Ice Beam Nidoking just for Gligar, which makes you resort to a claim of overcentralization fairly quickly. Now that Ice Punch is physical this is even a shorter step away, though Gligar can also overcome these moves a bit by the use of Roost. Plus it no longer sits around resorting to Toxic to bail out Waters - it can use Knock Off and Stealth Rock, which make it useful even if your opponent has an awesome counter like Rest/Sleep Talk Quagsire/Ice Beam Quagsire. It can Baton Pass, and fairly dangerous things too - Swords Dance and Rock Cart are nothing to be scoffed at.
I'd say Gligar being too good for UU in DP isn't such a far stretch. It pretty much made itself useless in Adv UU because even things with subpar Special Attack took up Ice moves for him. You've got to ask yourself if Gligar is having a positive or a negative influence on the UU metagame.
Generally speaking I am against the use of pre-evo's in UU out of principle. However, when debating issues I usually try to back that up a little more, and not just resort to "hey that defeats the purpose of playing UU". That means I should have to prove Gligar is overcentralizing UU, and so should you.
Personally, I think Gligar promotes a rather defensive metagame due to never dying with Roost, and is also threatening in a way Celebi was in Adv OU: even if you got something that stops his decent attacking assaults and you don't mind the utility he gives (in Celebi's case Heal Bell/Leech Seed, in Gligar's case Stealth Rock/Knock Off), there's always the chance he uses your switch to set up and Baton Pass away. And for Baton Passers, the closest thing you have to a counter is a phazer.
