Gathlain are a strange fey creature with ivy like wings. Their defining trait is their ability to fly.
Ability Scores: +2 Charisma, +2 Dexterity, -2 Constitution. Dexterity is always nice, but the constitution penalty hurts. The Charisma bonus also significantly limits the number of classes a Gathlain will excel at.
Type: Gathlain are fey, they can’t be affected by spells and effects that target humanoids, such as enlarge person and charm person. As most of these spells are either aggressive or can be worked around, this is a good thing.
Size: Gathlain are small, thus gaining their +1 bonus to AC and attack rolls in addition to the other penalties and bonuses.
Speed: Gathlain have the same base speed as humans.
Natural Armor: +1 to natural armor bonus helps make up for the -2 Constitution, placing Gathlain back on the front lines if need be.
Spell-like ability (entangle, feather step): Entangle is a solid spell and feather step is situationally useful, so it's hard to complain. Still, those 2 RP probably would have been better spent somewhere else.
Fly: A legitimate fly speed, starting at level 1. Besides not ever having to worry about falling to your death, a fly speed ensures that you can stay out of melee if need be and fly about the battlefield, casting spells or shooting your bow. Sure, it's only poor, but it's a fantastic ability to have.
Low-Light Vision: Sure, it's not dark vision, and sure it's rarely every going to come into play, but it's something.
We are looking for classes that benefit from a bonus to both Charisma and Dexterity, and can make the most of that fly ability. The -2 Constitution somewhat balances out with the natural armor bonus, so we'll consider Gathlain to have the same survivability to humans or any other race without either.
The straight Charisma casters, such as the Oracle, Sorcerer, and Summoner will all benefit immensely from the abilities. Bards, Paladins, and Clerics are all good here as well, particularly those of the ranged variety, as well as the Ninja. A Mysterious Stranger Gunslinger is an obvious choice, able to deliver death from above. The ranged Samurai will have a big advantage over the Cavalier, as a mount will somewhat negate the great fly speed.
A ranged Fighter can do well, but a ranged Barbarian will suffer from the Constitution penalty. There's nothing particular to recommend the Druid, Inquisitor, Ranger, or Monk, but leverage that Dexterity bonus and you can be well on your way. The fly speed is great for the Alchemist, but he's able to self-buff with it by level 7 anyway. A Rogue will appreciate the Dexterity, and could possibly make some use of the Charisma for feinting You can make a competent Magus with a Dexterity build as well. The Witch and Wizard are again going to enjoy the Dexterity and fly speed.
Overall, there's no real class that the Gathlain can't be at least passable at. Only the Barbarian keys off of Constitution, and everybody can appreciate a little Dexterity and flight.