Nope, it's not gross at all. It's called rimming and many people find it enjoyable to give and receive.

However, a lot of people also do not like it, or else have never tried it and aren't certain. You will definitely want to ask your girlfriend if she wants to participate in this activity beforehand. I actually suggest bringing it up in a conversation, outside the bedroom, about sexual fantasies. "Hey, you know what's something I would really love to try with you? (Insert activity you want to try in here)." That way there isn't a ton of pressure that would distract her from what it is you are doing.

If you are more comfortable asking in the heat of the moment you can, just be sure it's at a good time, i.e. a time when she won't feel a lot caught too off guard or feel too much pressure to say yes or no.