the first element is "gausta", the name of the whole mountain, the last element is the finite form of "topp" m 'top, summit'. the name "gausta" is probably a shortened form for "*gaustafjell" (gausta mountain), because gausta originally was the name of a farm beneath the mountain. (for the same development see dovre.) the old norse form of the name was maybe "*gautstǫð", the name is then a compound of "gaut" f 'flooded place', and "stǫð" f 'stead, landing place for boat'. (the farm lies at rjukan river.)