Because I have actually been re-reading Overlord in anticipation for vol. 11 next month, I have a more coherent arguement to present. That said,
Ainz, for starters, has minor reality warping via the spell 'Wish'. In the game Yggdrasil, it was simply an ability that granted a random effect, but upon being transferred to the 'new world' in which Overlord is set, it grants any wish that it's user desires.
"Ainz realised that [Wish Upon A Star] worked differently here than it did in YGGDRASIL.
When Ainz had first learned of Nfirea's talent, he had idly wondered if he could steal it with [Wish Upon A Star]. Now, he realized that he could have done it. In this world, [Wish Upon a Star] was a spell that would grant its caster's wishes. Although it would still expend XP, [Wish Upon A Star] was now a spell that could make the impossible possible. In addition, by sacrificing five levels — 500% of his XP, one could grant even more greater wishes." I still have yet to read up on Gilgamesh, but Wish Upon a Star alone should probably boost his power to roughly Ea's. The only thing that was shown to be able to stop 'Wish' is a world-class item. Which, to be succinct, it the crystalized version of a destroyed world. " The World Tree YGGDRASIL had countless leaves, but then a gigantic monster appeared which devoured these leaves, causing them to fall one after the other, until only nine were left. "
...
"What did those World-Class Items represent? They were equivalent to those other leaves — in other words, a single World-Class Item represented a world. Thus, the basic design principle of World-Class Items was that they possessed enormous power, and indeed, many World-Class Items were extremely powerful."
Based on what little I know of Ea, it has the power to destroy the world. Thus, I would put it roughly equivalent to World Items, whom are shown to have the ability to do the impossible; Things like erasing people completely and utterly from existence, altering the fundamental laws of magic, altering the universe, and a weapon that grows in strength infinitely.
"There was [Five-Element Progression], which could request the YGGDRASIL developers to change part of the magic system.
There was also [Ouroboros], which could request a more far-reaching change to the system than [Five-Element Progression]. Then, there was the most powerful World-Class Item, [World Savior]. Normally, it only had the power of an ordinary club, but it could grow in strength without limit. A single enemy with this item could conquer the whole of Nazarick, even at the height of its power with all its members present." Ainz himself posses a multitude of these items, holding eleven in total, two of which belong to a group known as 'The Twenty'. The twenty are one-use only, after which they disappear, but they have greater power than even those listed above, which are simply normal-class world items. "The items known as the Twenty were so powerful that they would vanish after being used once, which was why he could not bear to use them lightly, but instead treated them as trump cards. Ainz Ooal Gown proudly boasted possession of two the the Twenty. " It's unknown which of the twenty Ainz possesses, but he does possess them and one can assume they are extremely destructive. At the very least, possession of a world-item is suggested to grant protection from similiarly powerful artifacts. "he only way to protect against the effects of a World-Class Item is to have a World-Class Item yourself." So he's got that going for him. Aside from that, however, there's a world item which he possesses that is of particular note to this discussion; 'Picture of Nature & Nation', which possesses the ability to permenantly trap another in an alternate dimension. I'd say that'd render Gil unable to fight. For that reason alone, I'd say Ainz would win. There's more I could probably dig up, but being able to [possibly] resist whatever trickery Ea does besides blow up the world, reality alteration, and the ability to trap Gil in another dimension is overkill enough.