Minhagim always evolve. chazal don’t mention chanuka Gelt, neither do any Rishonim The minhag of gelt started later and like many minhagim evolved now the minhag is changing to Gifts (among some).
Is the mekor from Goyim? Probably (Though I’m willing to bet that Gelt came from Goyim too but no source at the moment)
does that make it assur? Well we have halachos for that go through Y.D. 178. It is quite hard to argue that it is assur.