Gray is NOT on par with END in terms of raw power. Slaying magic gives huge specific advantages against the creature it's meant to slay (as much as a Pokemon double type advantage, if not more), yet EVEN THEN the fight was just even, proving how Gray would've been stomped if END wasn't a demon.
Laxus isn't a demon and he has far more offensive power and speed than Gray, but I don't deny Gray is smarter and more versatile. So it shouldn't be a stomp, but Laxus still wins.
Gray did stalemate END Natsu though. During their fight he went toe-to-toe with Natsu in striking strength. He was able to knock Natsu down. and pushed him back. Vice-versa happened to him. If you want to use Pokémon-theory, then fire is super effective against ice. In FT, it is actually canon that ice's weakness is fire too. Chelia, despite being a God Slayer, couldn't compete against Dimaria's powers when the latter was in her god soul Chronos' takeover. It was only after she activated third origin release with Ultear's assistance to access her peak potential magic power from the future, did she manage to defeat Dimaria. She also required Ultear's help to be able to move in Dimaria's Age Seal too, a spell stated to be taught to her from Chronos. So being a god slayer didn't grant her the strength to resist "gods" powers either.
The same reasoning is applicable to why Gray, despite being an ice devil slayer was outclassed by Invel, a regular ice mage, at the beginning of their fight. Invel could parry Gray's attacks with bare hands, toss him around with his own attacks, and affect Gray with his cold/freezing capability. It was only until he was resuscitated, that Gray could beat Invel into a pulp. Similar to Chelia, he became more powerful during his fight. Rather than third origin release though, the source of Gray's newfound strength came from his power of feelings. The dragon slayers could hardly damage the dragons when F.Rogue summoned them in the GMG arc too.
There are quite a number of examples showing us that slayer advantage isn't everything. Magic power and physical attributes should both be bigger factors to determine the relative strength between characters. Gray should be in the same tier as END Natsu in those aspects to be competing with him on equal footing. The difference between them were portrayed in their fight to be imperceptible. It was certainly closer than Laxus vs Erza, or Natsu vs Gajeel in the 100yq. Re. Laxus vs Gray, present day Laxus has mastered red lightning. For Gray, he has stagnated in terms of feats. Though I believe he has grown more powerful but simply hadn't display them yet. Until that is proven, it remains a speculation. Laxus should beat Gray without much difficulty.
