He was not the strongest, he was only the most apt at fighting demons.
That's like saying Wendy was stronger than Mira during the Eclipse Dragon arc because she did more dammage than Mira to Zirconis. When it's clear she only recently surpassed Mira
Natsu couldn't even follow Gray's cast speed in Tartarus arc.
Maybe that is because of Silver's powers, but still. He had a similar moment that LFD Natsu had in Tenrou arc
Gray was probably the strongest member of FT in that arc, apart from DF Natsu
Gray was also one of the strongest FT members in Alverez arc, though that is attributed to his "darkness" mode.
He stalemated partial END, who in a less demonized (i.e weaker) form, overpowered Dimaria's timestop and no diff'd her. Gray also stomped Invel without even going into curse mark mode, while Invel previously comfortably dealt with Gray in his DeS mode.
In before "that is because of slayer advantage". Gray was on par with END in all stats. Strength, speed, attack power etc.
If he could one shot Mard with a clean blow, when Mard was leagues ahead of him back then. While END was tanking hits from him, while being relative in stats. That means the slayer advantage wasn't applicable against END, to anywhere near the potency it was against Tartarus members, that is possibly because Natsu was still mostly human and in process of transformation (though most probable answer is that Mashima didn't think too much lol)
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Point being, Gray was a real force back in Tartarus/Alverez days. He was treated with respect, truly worthy of being Natsu's rival, in the main series.
This is reflected in how majority of the people saw him back then. Look up the "FT strongest character" lists and videos during the arc, Gray was consistently put alongside Laxus and Jellal, maybe even higher.
In contrast, he is treated like dirt in the spinoff. Even worse than side characters.