I tend to go through this by figuring out what the character is not and then settling on a choice. So on what he isn't:
Gale can be reckless and seeks adventure, he hates to be stagnant and still, but I wouldn't classify him as brave or even particularly daring. His chivalry is surface-level at best, which crosses him off of Gryffindor.
While Gale is inherently friendly and accepting of all types, loyalty is a primary characteristic of Hufflepuff and he'll throw most people under the bus to save his own skin.
Gale likes to know things, and he's smart, and he's witty, but he doesn't value the pursuit of knowledge or the pursuit of a formal education in the way that a Ravenclaw would.
So Gale is a Slytherin. While he doesn't desire power and doesn't consider himself ambitious, he prioritizes resourcefulness, cleverness, determination, and cunning over other values. He'll betray you to save himself, except in extremely rare circumstances. He's not going to sacrifice himself to save the day, he's not going to act like that kind of nobility is inherently better -- it's stupid. Self-preservation is Gale's dominant trait, and he'll do almost anything to stay alive.
Incidentally, the Scarf of Sexual Preference says Heteroexceptional.