class Typable x => Term x where gmapT :: (forall y. Term y => y -> y) -> x -> x
In "gmapT f x" the function f must be of type "forall y. Termy => y -> y". That means that for EACH type y which is an instance of "Term" class, f maps y to y. You can't do something like "gmapT (+ 1) x", since (+ 1) is not general enough. However, if, say, Term class provides a method class Term a where ... pnt :: a ... then you CAN do, for example, "gmapT (const pnt) x", since the type of "const pnt" is general enough (and even more general).
At this point, I am at a complete loss as to how this is any different from
gmapT :: Term y => (y -> y) -> x -> x
Here you CAN do "gmapT (+ 1) x", assuming that, say, Int is an instance of Term class.