To compensate for the new sub-factions, GDI has been bolstered particularly in the area of its air force, which has more firepower (compared to its sub-faction cousins) and can utilize support airfields. GDI also has the best AA capabilities and can build the devastating Shatterer tanks.
Units

SlingshotThis hovercraft is a sole anti-air vehicle. It tears any and all aircraft to shreds within seconds, although it’s very weak in direct combat.

ShattererThe Shatterer is another hovercraft unit armed with a sonic emitter cannon. Use this unit to smash through tank spam and bases. The ZOCOM Shatterer can unleash an extra damaging Sonic blast which does insane damage but shuts down the Shatterer for a few seconds.

HammerheadHammerheads are probably the toughest air-craft GDI can build and is great against infantry. They can be garrisoned with infantry units for added versatility. Hammerheads never have to return to the airfield to reload or refuel, and you aren’t limited in the number you can build per airfield.

MARVGDI’s epic unit. Armed with a tri-barreled gun that fires sonic shells, this beast can be “garrisoned” by four infantry units. When garrisoned, the infantry battalion is sacrificed and one of the MARV’s hardpoints is upgraded with the infantry’s weaponry. The MARV can also be used to harvest
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Upgrades

HardpointsHardpoints give your orcas and firehawks fifty percent more firepower, increasing the orca load-out to nine from six rockets, and six from four missiles or three from two bombs for firehawks.

Armor-Piercing AmmunitionYour riflemen, hammerheads and APCs deal extra damage against enemy vehicles. This could be particularly useful for hammerheads, allowing you to harass enemy harvesters.
Tungsten AA ShellsThis upgrade improves your AA machine guns by increasing the damage done by them.
Buildings

Reclamator HubThis structure is required in order to build the MARV. It can also build vehicles like a normal War Factory. Naturally, you are limited to only being able to build one of these.
Command Support AirfieldThis new building is essentially an airfield without the ability to build aircraft. It can repair and arm aircraft, as well as open up your airfields so that you can build additional orcas and firehawks. Since it is a defense que structure, it also makes a good emergency landing pad should your original airfield get destroyed.
Support Powers

Sonic Repulsor FieldsThis power is cheap and recharges quickly, but can only affect one building at a time. The targeted building becomes immune to engineer capture (unsure about commandos and shadow teams, but it would make sense).

Thor’s HammerThis is GDI’s new base-breaking power. Your enemy will be warned by a laser beam that precedes the attack and be able to move his units in time, so it’s mostly useful against bases.

Supersonic Air AttackCalls in several supersonic fighters that quickly eliminate all enemy aircraft within the area. It takes a couple seconds to arrive, do don’t waste it on moving air units!