Yes. I have a girl friend. Is grammar correct?

The grammar is technically correct, but it sounds a bit awkward and informal. Here are some better ways to say it:

* "Yes, I have a girlfriend." This is the most natural and common way to say it.

* "Yes, I'm dating someone." This is a more general way to say it, without specifying the gender of your partner.

* "Yes, I'm in a relationship." This is a more formal way to say it.

It depends on the context of the conversation and who you are talking to.