If Gay can have enough longevity as a Rams kicker, and keep up this kind of form, I don't think it will be close. He will go down as the franchise's best.
He is very accurate, and somehow, extremely quietly, has provided a leg almost as strong as Greg Zuerlein's league-renowned boot. Gay is hitting from well into the 50s with regularity, and many of those kicks would be good from 60+.
I think part of the issue is that kickers seem to be improving in the NFL. The FG attempt range that is deemed "risky" seems to be getting pushed back a few years every couple of seasons. Even as recently as in the Fisher era, if Gay was putting in this body of work, I think it would be getting a lot more talk. I wonder if in another few generations, the NFL will have to prohibit FGs past a certain distance, as I cannot see them changing the field dimensions and it would seem ludicrous to almost always be in FG range.