The Shadow of the Past - Gandalf's knowledge

[JeyEn]

Hello everyone!

So after rereading chapter II of book I, I stumbled upon something. Gandalf said he knew from the beginning Bilbo has a Great Ring (as plain it was). This obviously isn't corresponding with the favourite theory of lesser ring. No, Gandalf knew from the start he is dealing with Great Ring. But later in that chapter, he says all Great Rings are accounted for.

This "problem" just bothers me more than it should. If dealing with a Great Ring, shouldn't Gandalf do something about it? And not wait 60+ years. Why didn't he ask Elrond for advice? I understand that Bilbo seemed fine with the ring and knowing its location was probably for the best, but still - by the process of elimination, he could have started to suspect Bilbo has the One much sooner, no?

And why was Gandalf concerned by Bilbo not aging? He knew he has a Great Ring - and these rings prolongs life. It just doesn't make sense to me. I tried to ask on reddit, but nobody answered without the "lesser ring theory". Any help? If someone could explain to me this, I would really appreciate this. Thanks!