It depends on the conviction and bond you share with your gf.
If you are really serious about marrying your gf, you should at the very least let her know about this other gal coz in a relationship, it's about mutual trust and understanding.
Personally, if it's me, I will have to sit back and think how I would want things to turn out. I'm very sure you have a slight answer within yourself on how you want things to turn out. The reason why you are asking this question is that you want somebody to answer for you, what you already have in mind. So don't blame me for being frank, but you just simply have to decide yourself.
Me, if I have a lasting relationship with my gf and am already thinking of marriage, I will drop all other notions and ideas.
Hope this helps.