Goals from Matt Phillips, Alan Bennett and Kevin Betsy - together with wins for Exeter and Tranmere and a draw for Hartlepool - condemned Mark Stimson's men to an instant return from League One to the basement division on Saturday.
After Wycombe dominated the first half, Phillips brilliantly put the Chairboys ahead on the stroke of half-time when he cut in from the right flank and curled a delightful left-foot shot inside the far post from the edge of the box.
Gillingham blew three golden opportunities to equalise as Rene Howe lobbed wide of an open goal, Simeon Jackson volleyed John Nutter's cross wide from six yards and Dennis Oli scuffed a shot straight at keeper Tom Heaton.
Bennett doubled the Chairboys' lead against the run of play in the 64th minute when he headed in Phillips' corner at the far post.
Wycombe striker Jon-Paul Pittman had a goal disallowed for offside before Betsy sealed the win when his angled drive deflected off defender Mark Bentley and beat Alan Julian at his near post.