Tweaking Underwhelming Characters.

More people getting into command hacks is a good thing. That said, commands can be kind of complicated because you are writing code in two different sections, C1 (the visual side of battle code) and C2 (the mechanical side of battle code). It's not always clear how the two portions of code line up. I don't know how much or little the "replace possess with an 8-spell command" does for you in terms of setting this all up without looking at it myself, so maybe it wouldn't be all that hard if you're using that as a jumping off point.

Giving Gau a desperation shouldn't be too hard. You'll need to find a free attack (I think there are 1 or 2 "unused" attacks in the game) and then edit the function for Desperations so that Gau can do it, then make sure it selects your new one for him, since all the other ones are derived mathematically based on their character ID number (which wouldn't be the case with Gau as you are replacing a random spell ID with a new desperation for him).

These are good ideas. I would say pick one thing at a time and build from there. It's the best way to learn hacking. Maybe first figure out how to edit spells to make a Runic spell... which is actually another that would need to replace an existing spell because the Runic "attack" is actually built dynamically in C2 rather than taken from the spell data block.

Before the last time my hacking trailed off, I was prototyping command and attack expansion for this very kind of thing. It can be a bear, for sure... which is why these kinds of changes appear generally only in extensive hacks where a large portion of the spells are changed already so it doesn't conflict with vanilla enemy scripts or commands.

Good luck and Happy Hacking!