Well, we have to be clear here about what a gambit is, a gambit is the intentional loss or sacrifice of a pawn or two. The queens gambit and vienna gambit are definitely both "gambits" you give up the c pawn in the queens gambit and the f pawn in the vienna. Now with your reasoning that they arent actually true gambits because you are garenteed to win the pawn back, they are still gambits, and your GOAL is to win them back but black can still hold onto them in lines like the greco variation with QG. All "Gambits" are actually gambits or they wouldn't be called so.
gambits that are not really gambits


#2 1.d4 d5 2.c4 dxc4 3.Qa4+ winns the pawn back instantly. The fact that white has even better options is irrelevant in this context. So 2.c4 is just offering a pawn exchange. It may be a real gambit after some moves if White don't try to get the pawn back, but 2.c4 is in my opinion not a real gambit.
I've heard about the Queen's Gambit and Vienna Gambit that they are not "true gambits" (i.e. the opponent cannot prevent you from winning the pawn back)
do you know other gambits like this that are not true gambits?