You can catch Gligar at Route 206, Route 207, Route 214, Route 215, Route 227 and Stark Mountain. However, you must have Pokemon Emerald inserted in the GBA slot of your DS.
If you have a DSi or a 3DS, you're out of luck here and you cannot catch Gligar. Sorry.
If you want to transfer Gligar over to later generations, you can catch some at Route 206 in Platinum, Routes 11 and 15 in B/W, Routes 11, 15 and 23 (and the Hidden Grotto in Route 23) in B2/W2, and Route 19 via horde in X & Y.