Rages are learned by using GAU's 'Leap' command in combat.  Once you have lept, you will exit combat with your party minus GAU.  After one or more combats, GAU will find your party again with new rage technique(s). You will learn rages for all monsters in the initial combat and for the
combat in which GAU returns.  LEAP CAN ONLY BE USED ON THE VELDT!  The Veldt will contain (virtually) all of the monsters you have encountered so far in the game (so return here after you have fought new monsters). A few monsters, mainly bosses will never appear, though.  Also, there are a few monsters that rages cannot be learned from (possibly a bug in the
game).  So far, these include ProtoArmor, Pugs, and I believe one other. No matter how many times you Leap at them, you will never learn their rages.

Rages, once initiated in combat, cause GAU to go into a berserk-like frenzy, which cannot be broken - you no longer control him.  The only way to stop a rage is upon the end of combat or by dying.  Each rage has a normal attack form and ONE special power--ONLY ONE, even if the monster had several.  The CPU will randomly have GAU attack or use his special power each time he gains the initiative.  The special powers vary widely in effect and usefulness, and often replicate spell effects (but at no MP cost).  Rages can also give protective/enhancing spell effects inherent to the monster.
 
There are 256 slots for Rages, and I think there are 255 total (the magic number for this game!) Here is a list of the special powers produced by Rages (there are still a few missing).  This list is divided into 16 tables of 16 rages.

KEY
*  Indicates a special power that can be learned by STRAGO as a Lore effect
:) Indicates one of the most useful techniques (in general)
:( Indicates a technique that could prove harmful to GAU or the party
   (be careful about using these!)