Covenant Wins.
Gordon would pwn Chief, because, let's face it, a Grunt could do that. But the Covenant would pwn the Combine. Before you disagree, let's look at the facts.
1. The Covenant can use portals to travel through space within one universe, but the Combine can only travel between universes. This would give an advantage to the Covenant, if the Combine were to attack first. Which they would, because the Covenant can't travel to their dimension.
2. The Combine took Earth in 7 hours. The Covenant vitrified reach in two and a half hours. I repeat. Vitrified. Reach. Two and a half hours.
3. The Covenant have the Mgalekgolo, more commonly known as Hunters. For those of you who don't know, imagine the Hulk. Give him indestructible plate armor. Give him an indestructible shield. Also, give him a Covenant Assault Cannon. (Mind you, this is a Covenant Assault Cannon. Range: Long. Accuracy: High. Fire rate: Continuous stream of incendiary radioactive gel). Then, give him near-psychic abilities. And just to rub it in, make at least a billion copies of him, and make them travel in pairs.
4. Covenant weapons are much cooler, and are very effective. Their
weakest weapon melts your skin off. Their strongest ones melt your planet off.
Covenant wins.
P.S. Gordon has ONE Gravity Gun. The Covenant has Gravity Hammers. Plural. And Gravity Cannons. Also plural.