Rifleman: 300 Tiberium
The rifleman squad is the basic combat infantry for the GDI. They are your grunts use them in the earlier missions but Snipers and rocket teams are far better when used together. Only use I found for them is their ability to build bunkers which can be garrisoned by rocket men to turn them into excellent AA implacements
Missile Squad: 400 Tiberium
Missile squads are your tank specialists. They are fairly useless against infantry but when garrisoned with a sniper turn into the best form of base defence.
Sniper Squad: 1000 Tiberium
Perfect anti personel unit. See above for best application.
Grenadier Squad: 800 Tiberium
These units should be used in the first few missions to clear out buildings but after this your armour should be able to do the job.
Zone Trooper Squad: 1300 Tiberium
The zone trooper squad is your most powerful infantry. They use rifle-sized railguns, giving them an absolutely deadly (one shot, one kill) attack against infantry, and deal serious damage to vehicles. With upgrades, they can also detect stealthed units, making them a great support unit for a vehicle attack. Their jetpacks are useful for surprise attacks, but be careful where you land them: they can't fire while jetpacking, and have a cooldown before they fire once they hit the ground. If you aren't careful, you can lose a ton of them by jumping into a group of enemies. Of course, if you are careful, you can zone tro
Commando: 2000 Tiberium
The commando units are an oddity. They are useful in certain situations, but I generally avoid them. They cost 2000 Tiberium that could be used to buy things that will probably serve you far better than a single commando. If you have Tiberium to spare, and want to play around with a cool unit, go for it. If you don't, consider not using the commando. Also, remember that you can only build one, so consider how useful this single unit will be in your overall game. Of course, that one commando can destroy building and do nasty damage to small groups of infantry, so it may be useful in a tight game.
Engineer: 500 Tiberium
Engineers capture buildings. That's pretty much their whole use. They have no weapons, and will die very quickly if they come under fire. Use them to take over neutral building like tiberium spikes, and to steal enemy buildings. Interesting tidbit: For harder AI levels, when an engineer gets close to a building, the enemy will sell the building. If you can get an engineer into the center of an AI base, and they may very sell off their whole base to stop you from capturing it!
Pitbull: 700 Tiberium
Pitbulls are basically a missile squad on wheels. They also detect stealth, a useful ability for any combat group. Use them with APCs to create a fast, all- around effective combat group that can engage everything in the game. However, make sure you keep them out of serious trouble, because, as with all light vehicles, they are very fragile against serious anti-vehicle fire. If you're going to use them in an assault, or extensively in any way, you should invest in the mortar upgrade. It is a very cheap and effective upgrade.
APC: 700 Tiberium
APCs are, in my humble opinion, one of the best GDI vehicles. They can hold one infantry squad inside them, but more importantly, the unit inside can fire out of the APC. So, though the APC only has a machine gun effective vs. air and infantry, an APC with a missile squad inside is effective against all targets, and in groups, can slaughter just about anything. Plus, they're more manueverable than the mammoth tank, and can hire air transports as well. APC's can also drop minefields, useful for blocking off bridges or other natural tight-spots.
Predator Tank: 1100 Tiberium
The main mid-game tank for the GDI can be toughie to produce, but can smack their Nod equivalents around with no trouble. Also, the railgun upgrade will affect these as well. Though with less firepower, two or three predators will be able to engage different targets, with more speed and maneuverability than a mammoth tank.
Juggernaut: 2200 Tiberium
Juggernaut are another oddity. If you have some well-placed sniper teams, you can use these to knock out a large group of enemy units cleanly and quickly. Use them if the game is going slowly and you want to crack a tough defense. Some people use them for frontal assaults, but I suggest using them in more of a support role.
Mammoth Tank: 2500 Tiberium
The GDI main battle tank. A few of these will crush through most opposition, and it will take some actual tactics to take them down. With the railgun upgrades, mammoths will really come into their own. However, you must support them or lose them. Vertigo bombers will pound them into dust, so make sure they can't. Rather than building ten mammoths, build two or three mammoths, some zone troopers, and a couple APCs with missile squads. This group is more versatile, can move better through small spaces like city fights.
MCV: 2500 Tiberium
Not a neccisity because of the Surveryor/Outpost, but if you want a real backup base, you need one of these. It's totally unprotected, so make sure to send and escort if you really need it to live. When deployed, this will create a construction yard, the building you need to start doing, well, anything!
Rig: 2000 Tiberium
An interesting unit, that creates a battle base when deployed. The base is equipped with some turret weapons, and can repair nearby vehicles. It does not create a build zone around it, but pairing a rig with a surveyor lets you create a defended expansion base quickly, and you can even pack it up and put it somewhere else.
Surveyor: 1500 Tiberium
Surveyors are used to create outposts, which generate a build zone, allowing for the creation of expansion bases. Totally unarmed, so you want to defend them if you're in the thick of the battle.
Harvester: 1000 Tiberium
Harvesters... well, harvest. They have a light machine gun, but it's pretty much useless, so defend your harvesters well. You can build more from the war factory, but refinerius come with a free harvester as well. Note though: if you make a refinery, then sell it, you get a harvester for 1000 tiberium, the same as it's individual price at the war factory. Useful for getting extra harvesters in the early game.
Orca: 1100 Timberium
Orcas are like small nuggets of goodness, filled with rockets of course. Orcas are excellent support vehicles: bring them in to support tank charges or to counter-charge enemy tanks. A group of twelve orcas is easily maintainable, and will rip tanks and even infantry to shreds. Try not to attack with only orcas though, because they are very vulnerable to anti-air fire. Also, after you research the sensor pod upgrade at the tech center, orcas can drop small sensor pods that will detect stealth and reveal the area around them. These are a must buy.
Firehawk: 1500 Tiberium
Firehawks are your bomber unit. Upgrade to these late game, but don't stress it if you don't have the money. Firehawks are very fragile, and can only fire one salvo before needing to return to the airbase. Of course, with their special upgrade, they can travel anywhere on the map instantly, and drop their devestating bombs to cripple enemy bases. However, DO NOT USE FIREHAWKS ON ENEMY UNITS. You will fail miserably. The bombs fall too slow to hit moving targets, and don't deal enough damage to truly disrupt enemy attacks. Use them to destroy enemy bases, twelve firehawks, split into three groups of four, can kill or seriously damage three buildings. AND, they can drop directly into the enemy base, right on top of that building. Remember though, that the cooldown means you can't stratosphere fighter your way back out, so you may lose some on the way out if there is a lot of AA around the enemy base.