So a spell that targets a creature should say it affects the gear or equipment carried or worn, or it would not. This is supported by many spells that are explicit about affecting the target creature's equipment, for example invisibility and polymorph. But banishment has no language about carried or worn equipment, so it would not include gear. (Neither, by the way have teleport, plane shift or imprisonment, which would mean they transport their targets without clothing or equipment, Terminator-style.)
Everyone I know plays these spells as transporting the creature with all its gear (for example, this question assumes it). Banishing otherwise would be powerful against opponents that rely on equipment to fight: you could banish them and have them come back without it and relatively helpless; teleporting otherwise would suck.
Does banishment remove the creature and leave it's equipment behind?