i dunno, because when i tried it, GGN was not great at all, it can only use the TMS weapons, which means you've already played as him before if you equipped that on other mechs, its SP attack is subpar, granted i didn't max it out before having a final comment, but it didn't take maxing a unit to know that it's good, i've used fresh units in hard missions before and they hold out just fine(taubarn, billbine, ACR 10, konigmonster, etc)
becoming weapons for others that doesn't have projectiles like Heroman i guess
you see, the TMS(short for TAMASHII) weapons that you get/unlock in the game are the limbs of GGN, the two feet are missile launchers, one of the arm is a beam launcher and the other a gatling gun IIRC
after you've beaten ideon you get the main body of GGN and hence, GGN is unlocked for play since all of his parts are complete.
i don't remember for sure but if you look at the picture of the GGN figure that sephia wind posted in page 1, you can see the ARX-7 arbalest using the TMS beam launcher and the mark of the weapon is the same as the one on the right shoulder of GGN, which means that its his right arm
that means the left arm transforms into the gatling gun(which does not feel like a gatling gun, the ACR 10 from armored core V has the real feel and effectiveness of a gatling guns in the game and is one of the best units overall IMO.)
the only slightly decent GGN limbs to actually be used by other characters is the beam launcher for those who doesn't have a similar weapon or their main projectile is worse than the TMS beam launcher(like heroman, which has no ranged weapon, and taubarn, which default beam shot is way slower and impractical to use)