No matter what she does from now on, even if she doesn't release one pop album from now on, she's a legend that will be remembered forever. The same can't be said for pop girls who have tens of #1s.
Just like David Bowie, while she didn't always slay the charts, she ALWAYS was and will be remembered. Why is that?
Is it her undoubtable talent? Her influence on the pop music scene? Or the quality of her music?
No she is not. A high peak in music doesn't make you a legend. She has a long way to go -- and her next album is going to either make or break her so postpone the legend talk.
Gaga, like every single artist that debuted this century, has a lot more to prove before we start calling her a legend.
It's safe to presume that she will be remembered as a legend in the future though, the impact is there, but it's a little to early to make this thread.