Is it ok for a teen boy to gift his teen girlfriend underwear?

No what kind of creeper are you. THat's disturbing and has a lot of implications that you shouldn't get into, I mean what are you trying to say about her underwear and why have you seen them?

But it obviously depends on your relationship, situation in your relationship and beliefs and of course how old YOU are, since if you're that much older than her, it makes it even worse. I personally do not think it would be ok.