I feel a young girl is falling for me, but I'm gay.

Well i met someone online, we chat but not all the time, and just making friends. But I feel that this girl is looking for more than just a friendly conversation. The problem is, I'm gay and not into romantic relationship with girls. I haven't told her that I'm gay because I think it's not important because I'm just there to make friends. Should I tell her? And BTW I'm not still out.