Recently Ganthet started using a power ring, but isn't he more powerful then a power ring so why does he need one?

Probably simply in order to be formally a part of the corps. If there is a better reason, we haven't heard of it yet.

I believe that he uses one in order to be a Green Lantern, he doesn't or didn't need it at the time, because during the Blackest Night he could use the ring, AND use his blue aura. Now however, I'm not so sure. It is possible that he has biologically locked himself from his natural powers. Guardians are capable of augmenting how powerful they are because they can manipulate their own biology at will.

"He just wants to fit in? "

"@longbowhunter said:

"He just wants to fit in? "

Dont we all? "

Because, some people don't know how to write the Green Lantern/Guardians properly.

 @DarkKnight96000:  To focus his power effectively. Plus despute all his power the power derived from the green lantern power battery on oa is greater 
 
Finally becoming a green lantern was a ruse to allow him to follow a different threat detailed in emerald warriors

You guys were all up his *ss when he had the ring... Now look at him! He lost his damn hand!