Battle Moveset

A Good Moveset for Gligar would have to be this:

Attacks:

Aerial Ace or Hidden Power [Flying]
Earthquake
Swords Dance
Quick Attack or Rock Slide

Items Attached:

Leftovers OR
Salac Berry

Preferred Nature:

Jolly

Strategy Using Gligar

Gligar is one of the better Underused Pokémon with very solid sweeping ability. It gets Sword Dance and has a very solid Defence stat and no defensive weaknesses. The basic idea is to switch into an Earthquake and Sword Dance up while the opponent switches to a Pokémon who can handle Gligar. Ice and Water attacks will leave a huge dent where Gligar's HP stat once was and with the abundance of Water Pokémon, many are classified as Underused, so you'll need to watch out for those when playing the UU Metagame (Slowbro especially, who has the Defences to switch into a Gligar's offence).

For something UU, it has many advantages. It gains STAB on both Earthquake and AA (for Cartridge battlers)/HP Flying, has access to Rock Slide and Quick Attack. Unfortunately, that isn't enough to make it useful in the Overused Metagame. Skarmory can easily switch into Gligar even after a Sword Dance and will freely proceed to lay down Spikes and then follow up with a Whirlwind. There are also many fast devils in the OU Metagame who can KO Gligar. Jolteons with Hidden Power [Ice], Starmies with Ice Beam and Choice Band Aerodactyls/Tauroses/Salamences can all leave their mark on Gligar. It has the defences to act as a semi-wall in the UU Metagame but against OU Pokémon that defence simply isn't enough.

For Gligar's trait, Hyper Cutter is a lot more useful than Sand Veil. Blocking Intimidate is a lot more important than a potential fluke miss (and unless Tyranitar is in use/has been used, there shouldn't be a Sandstorm around anyway).

The Salac Berry offers its obvious advantages, allowing Gligar to outrun almost all Pokémon (who haven't used stat boosts) however Leftovers is consistent and Gligar may experience moments where it needs to be switched in and out a lot.

When using the Salac, you could consider using Substitute/Endure (in place of Rock Slide/Quick Attack) to activate the Salac Berry however I wouldn't suggest reliance on the Salac Berry. Gligar's speed stat is usable as it is and although it is outrun by a handful of Pokémon, it outruns the majority of Underused Pokémon.

Over all, Gligar is a solid performer in the Underused Metagame but it has major problems against Overused Pokémon.

On another note: This set works nicely for getting through the game (although remove the Salac Berry) as well as working well competitively.

EV Corner:

Jolly Nature (+Spd, -SAtk) EVs: 6 HP / 252 Atk / 252 Spd

These could be tampered with to add some more HP and/or Defence but you defiantly need to maintain maximum Speed. Attack can be sacrificed but make sure you keep the stat at the 200 area so that after one Sword Dance it becomes a very solid Physical sweeper.

As little note on IVs: When setting Gligar's IVs make sure to make all the IVs 30 except the Speed IV, which should be at 31. This leaves you with a 70 Base Power HP Flying as well as maintaining maximum speed.

Other Optional Sets: Gligar@Choice Band
Earthquake
Aerial Ace/Hidden Power [Flying]
Rock Slide
Quick Attack/Sludge Bomb

Sludge Bomb isn't really necessary since you have HP Flying/AA but it's a move it has access to, however I'd say that Quick Attack has more value. The Choice band set should follow a similar EV formulae to the Sword Dance set, focus mostly on Attack and Speed.

Strategy Against Gligar

Pretty much anything faster with Ice Beam or Surf. Sticking a physical wall like Skarmory (or the UU Slowbro) in its way will bring it to a halt. Defensive Water types can cause Gligar major problems and stat boosted attacks (regardless of being Special or Physical) will be able to hurt it.

If you lack an Ice or Water attack (and you shouldn't but let's say you're Ice/Water user has been taken down) then simply use STABed Special Attacks (obviously excluding Electric typed Pokemon, but who would they be out there ready to take a potential Earthquake?).

Will-o-Wisp is a very welcomed move to use against Gligar as well as non-Thunder Wave paralysis inducing manoeuvres.

Contest Moveset

A good Contest Moveset for Gligar would have to be this for the Tough Contest best with Impish, Lax, Relaxed or Bold Nature:

Attacks:

Swords dance
Rock smash
Aerial ace
Rock slide

Items Attached:

Yellow Scarf

Strategy Using Gligar

If you do the attacks in this following order you should have very few problems; 1st - Swords dance, 2nd - Rock smash, 3rd - Swords dance, 4th - Rock Smash, 5th - Aerial ace/Rock slide

Locations in Games

Ruby/Sapphire/Emerald Trade from C/E (R/S) Safari Zone (Emerald) Colosseum Snatched from Hunter Frena in The Under Fire Red/Leaf Green Trade from C/E

Animé Appearences

Gligar has had several Animé Appearences. First it was owned by a man who took on the persona of Gligarman, a superhero who fights crime with his Pokémon Gligar. Next one is used by a trainer in the Hoenn League Episode 140: The Superhero Secret! Episode 404: Opening! Evergrande Tournement