If some thought Gray was above Erza... I can't help you. Erza has the 2nd best POF in the guild. "Darkness Gray" was nothing to her when you see the incomprehensible strength she showed against Irene. Even at Tartaros Erza was slightly stronger than DeS Gray.
I think it's the fault of the fake "Natsu's rival" portrayal tricked Gray's fans into believing he was above the S ranks and even Gildarts.
I think that's why he gets abused in the sequel. Gray was very high and many saw him as top 3 in the guild and Hiro wanted to correct it... At a point when even Wendy and Lucy got more spotlight. Poor Gray. He doesn't even hold a candle to the current Erza and Natsu anymore.
Nah dude, this was pretty much the consensus during Alverass arc. You can go check powerscaling videos made at the time. I wasn't in mangahelpers at that time, but Gray above Erza was pretty much consensus in google plus as well.
And really, it isn't hard to see why.
A less demonized (i.e less powerful) version of END than what stalemated Gray, no diff'd Dimaria and scarred her for life.
After "darkness" came, Gray low diff'd Invel without even using his slayer marks.
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I don't think Mashima thinks in terms of "fans overrate Gray, so I shit on him"
More likely scenario is that what mashima wanted for Gray as a character itself changed. In the main series, Gray being a rival to Natsu was actually believable. He was also clearly being set up as the antithesis to END, Gray being made into a demon slayer was precisely Mashima making him a counterpart to Natsu being the demon END.
But.........somewhere along the line, Mashima dropped the entire END plot! After building it up for years.
So, at this point, Gray serves no real role.
What I'm trying to say is that, back then, Gray was likely intended to be as powerful as the majority thought he was.
And the likely reason Gray is shit right now isn't because Mashima wanted to stick it to fans thinking highly of Gray, but rather because gray being a legitimate rival to Natsu is no longer a view Mashima holds.
Fans don't come up with stuff randomly (except fanboys, but I'm not talking about them). They just see what Mashima is writing and get a feel based on portrayal and feats.
Which is why it is also consensus right now that Gray is weak, while the consensus back in Alverass was that he was strong