General

How much should i gain to suprise her?

Its going to take a while so don't expect any reaction real soon.
4 years

How much should i gain to suprise her?

Have you ever been bigger than you are now? If so, you could pack weight on pretty fast (apparently regained weight can even look a bit more puffy, too). How long do you have?

Maybe just make sure you wear something tight, and/or be a little bloated when you see her.
4 years