Though their feats are not aplenty in the Alvarez invasion arc, they are still mostly incredible.
Natsu sent a a guy with towering physique into the skies with a regular punch, with enough strength to blast him all the way through Ajeel's main airship.
When scaled to the height of the FT guild building, the fleet of airship are seemingly several hundred metres above ground level. This should put Natsu's striking strength far beyond building destruction, and on city level.
During his fight with Gray, Natsu had developed giant claws and his attacks were shrouded in dark flames. In conjunction with his heightened emotions, these should increase his offensive power. He shot himself towards Gray in a clash of blows. And despite the acceleration and momentum Natsu had on his side, Gray could negate and cancel out Natsu's punch with his own while remaining static with solely his strength.
As one would require at least an equal amount of force to nullify an incoming attack, this should reasonably place Gray's physical and striking strength in multi-City level.
This can be reinforced when Gray easily defeated Invel, despite the latter capable of covering Fiore in glaciers.
To be fair, Fiore according to Zeref's estimation was reduced to 1/20th of it's original size after Irene casted Universe One.
Regardless, Magnolia is one of the twenty cities introduced in Fiore. It's safe to surmise Invel has the magical capacity and energy to affect the entire city of Magnolia, if he could do so to a shrunken Fiore. Gray could overpower what Invel self-proclaimed to be his strongest spell, so anything below multi-City for Gray's power and damage output would be hard to accept.
Gray has grown and developed into one of the strongest characters from Ishgar's side, and he could give Laxus a run for his money with regards to strength, stamina and endurance, and pose a credible threat. The guy has suffered grievous wounds, and ignored them to continue fighting. Gray's consistency should place him in the highest echelon when it comes performing despite debilitating condition. But then again, so does Laxus. And in Laxus' argument, he is even more rounded out when it comes to attack potency. His fight with Wahl shows Laxus displaying a sharper degree of reaction and velocity, which puts him ahead of Gray and Natsu, and an even more dangerous force to be reckon with.
Gray is powerful, but still a man. Laxus is a friggin' beast. And it wouldn't be fair to see him not even making it to the last 8. So he gets my vote.