Although technically any match of any kind of garter belt with any kind of sock is a "garter belt sock", this term is better reserved here for regular, below-the-knee socks being held up by garter belts worn at the legs as opposed to at the waist.

post #3106121

More detailed explanation:

Whereas garter belts are usually worn around the waist and are strapped with stockings or other female thigh-high socks (sometimes even in a one-piece fashion, see garter belt leggings), they can also by worn at each individual leg and be strapped to either a thigh-high sock or a common one i.e. which ends below the knee. Whereas the former is often seen as enticing as with the waist-level garter belt and should be tagged as such, the latter is usually considered as purely practical and maybe even comical-looking, being mostly associated with formal menswear.