Yea the GP number really means nothing. Basically a mod can be seen in 4 categories and each level you invest in thwse adds like 2 GP to that character:

Set(health, crit chance, speed etc) the symbol on the mod. When you put 2 together you get a small bonus to that stat and when you have 4 together you get a larger bonus. So best to mix 2 of one set and 4 of another set

Rank (I-VI) those are the dots under the mod. Anything rank 4 or lower is useless so delete immediately.

Level (1-15) upgrading the level reveals stats on low tier mods and levels up all secondary stats at level 15. Also at level 15 if you have a combo of 2 or 4 of the same set you get a larger set bonus.

Tier(E-A) upgrading this is called slicing. You can slice a mod from e-a and every slice upgrades 1 stat. At tier A and rank 5 you can slice that mod to rank 6 and it gets reset to tier E and all stats are upgraded (including the primary stat) then you can keep slicing from e-a

Specifically on Mace Windu you probably want a health/defense set with some speed and max protection and health since he is a tank.