Asking a girl out means to be taking her on a date, and hoping to take it to the next level; which is a full-on relationship. So, no. It's not the same thing. You would have to date her first, before you can call her a girlfriend. That is my own opinion though.
If you just want to go on a date try "would you like to go on a date with me?" or "would you like to go (insert activity here) with me?" so she knows it's referring to one outing, and not confused with asking her to be that special someone for multiple.