Galactus with the Power Gem vs the Marvel Universe
Who can stop Galactus with the Power Gem, without the aid of other powerful artifacts (i.e. other Infinity Gems, Cosmic Cubes, the Ultimate Nullifier, etc.)?
The Power Gem by itself would grant him increased strength and durability. That being said, higher-level Celestials such as Exitar, Celestial Hosts or possibly, even Necro-Thor, still, would be capable of defeating him.
Mad Celestials are only equal in power to individual Celestials, such as those in the Fourth Host. Therefore, an entire hose would be superior to the likes of Four Mad Celestials and Exitar alone, would be superior to entire host's of Celestials.
Galactus still wouldn't be capable of taking on entire host's of Celestials, even with the Power Gem, let alone a Celestial such as Exitar.
The Mad Celestials destroyed a universe with multiple Infinity Gauntlets used by the Council of Reeds, and yet a decently-fed Galactus was handing them their asses, killing one, before he paused and they were allowed the time to combine into one gigantic composite being.
The Power Gem would eliminate Galactus' hunger and constant need to replenish his stores. His power and durability would be amped to an unimaginable degree, and would never drain. If a recently fed Galactus can kill a Celestial as easily as he did, Galactus with the Power Gem shouldn't have too much trouble with all 9 Celestials...10 if you include Exitar.
The Mad Celestials are stated by Hickman to be equal in power to 616 Celestials, who created the Marvel Multiverse by splitting the Universe into separate parts, destroyed Universes and intimidated Kubik, who himself held a Universe in the palm of his hand.
Nothing infers that the Mad Celestials are anymore powerful than the Celestials from the Universe of 616.
Nothing infers that the Power Gem would eliminate Galactus' hunger and nothing supports that.
The Power Gem is a nigh-unlimited source of power...
Galactus typically only consumes what he needs to restore his power and survive, meaning he's running near empty most of the time. The exception is when he prepares for a fight, like the did with the Mad Celestials, who he clearly had the upper-hand against until they merged into a composite being.