Is it wrong to not want to give my girlfriend the profits from a watch she gave me?
Why are you not willing to share the profits?
That watch was her gift to me.
You should always keep the gifts people gave you. It's frown upon to sell gifts and makes your look ungrateful.
Nah, I don't want to keep this gift.
It will upset your girlfriend if you sell her gift. You could give it back to her, maybe gift it to someone else, or forget about it entirely and just ignore it. Selling it should be a last resort since it is essentially slapping her in the face.