Well no much to say, here are all the GF's including locations, and some other none GF's or Semi-GF's.

QUEZACOTL & SHIVA
Quezacotl and Shiva can be obtained in two places. One is when you first finish in the classroom, you can go to your study panel and get them there. Or, you can go to the frontgate and Quistis will give them too you.
IFRIT
Ifrit is one of the GF, like Quezacotl and Shiva, that you have to get. You just go to the Fire Cavern (East of the Garden) and beat Ifrit to obtain him.
SIREN
After beating Biggs and Wedge at the EM Tower, you fight a monster, Elvoret, here you can draw Siren.
DIABLOS
The next GF you can get is Diablos, though you might wanna wait for this one. Okay, on your second mission (the one with Rinoa and the Forest Owls) you will get a password from a Garden Faculty guy, when he gives you the password, that is when you talk to Headmaster Cid. He gives you a Magical Lamp, you can use it to go another dimension. Beat Diablos to obtain him.
BROTHERS
To get Brothers you have to go to The Tomb of the Unknown King. You will automatically be sent to the Tomb but you don't have to go in to far. If you chose to go in you can go to the right side and beat Sacred. Then go to the top part, flick a switch. Then you have to go to the left side, click a button and then go to the very middle and beat Sacred and his brother, Minataur.
CARBUNCLE
To get Carbuncle all you have to do is, on the fight at the end of disc one (the fight with Iguion, right before Seifer and Edea) you have to draw Carbuncle from Iguion. Simple eh?
LEVIATHAN
Leviathan can be obtain by drawing from NORG in the battle of the Garden, right after the Missle Base party gets blown up.
PANDOMONA
Pandomona is also drawn, drawn from Raijin in Balamb City, in the second battle, the one with Fuijin.
TONBERRY
Tonberry is actually hard to find, 'cause what you have to do it go to Centra Ruins (in the South) and defeat about 10 tonberrys. Once you beat roughly ten, right after the last battle a Tonberry King will show up. Draw Full-Life like mad then do Renzokuken, over and over. Then you get Tonberry.
ODIN
I put in Odin 'cause Odin is actually a GF, you don't get him to junction, but if you scan, you can see that it says his is a "legendary GF". Now all you have to do it go through the Centra Ruins to the top get the code, then re-enter it near the door where Odin awaits. If you are having troubles figuring all this out, Click Here.
CERBERUS
Cerberus is easy to get too. When under attack by Galbadia Garden, and you go in the Garden to attack the Sorceress. Go to the middle of the Garden and you can see Cerberus, you have to click the action button to fight. But...if you go the other side of him, there is a save point. 'Cause if this is your first time you might loss.
ALEXANDER
Alexander is obtain by drawing it from Edea in the final battle with on disc.
DOOMTRAIN
Doomtrain is not like other GF's you have to get an item and then more items. First you have to go to Tears Point. Go up to the Statue, and right near the statues right foot if you click around you should be able to get Solomon Ring. Once you have the ring you have to get 6 Remedy +s, 6 Steel Pipes, and 6 Malboro Tentacles...To get remedy +s you have to have a certain Ability (Sept-RF) to refine 10 Remedies. Steel Pipes can be stoolen from a Wendigo, the Wendigos can be found between, Deling City and Tomb on the Unknown King. Hmmm...Malboro Tentacles, I think can be refined from the Malboro Card with Card Mod (Quezacotl Ability) or by defeating on. Find them in east Esthar in the area of trees. Or Island closest to Hell/Heaven.
CACTUAR
Have you ever notice a big cactus popping up on a dessert island. Yea sure we all have, anyways, once you have the Ragnorak, go to the island and run into the cactus, really a Jumbo Cactuar, defeat it to obtain Cactuar.
BAHAMUT
Yes Bahamut, though very hard to find is in FF8, if you go to the Southwestern part of the map, as far as possible there is a island, actually a Deep Sea Research Center. You have to have Ragnorak to get there, you can go look at it with the Garden but you can't get in. Go in and make your way through til a question comes up. When it does you have one option pick it. Fight Ruby Dragon and draw some Metoer if you want. Then you get another question, answer Never, to fight another Ruby Dragon. Then last there are two options in the next question, answer the third. Yep go down and you cursor will go to a invisible answer, then fight Bahamut.
EDEN
To get Eden, you must defeat Ultima Weapon. Ultima Weapon is on the sixth floor of the Deep Sea Reasearch Center. Ultima Weapon is very difficult to beat if you do not have everyone at a good level and have the best magic. Here is a little strategy to help you.