Word for "when someone does something good for you and then mentions it persistently afterwards"

It depends on the gift giver's intent, but could he have been "holding it over" you or "lording it over" you?

Either way, he "never let you forget" that he once gave you that gift. And you may "never hear the end of it."

Milked it for all it was worth

Free Dictionary definition

never let them forget it

E.g.:

John once drove Jane to the airport as a favor, and he never let her forget it. Just the other day, twenty years later, he asked Jane to get him some coffee; saying, "Remember when I drove you to the airport? The least you can do is get me a cup of coffee."

'Harping on' is the term.

She bought me a nice gift/saved me some money/ did me a great favour - and cannot stop harping on about it.

Collins concise dictionary.