I did a bit of looking through the old v2 code to see how GCs work. Here is a bit of what I found:
Your GC's stats, skills, hp, and end raise continuously with your GC mod (some faster than others).
The exp of your GC is then calculated from how high these stats, skills, etc. have been set, and then the GC's rank is given accordingly. Thus ranks do not directly influence the capabilities of your GC.
Instead, and quite surprisingly, the GC's get some hefty av and wv bonuses each time they cross an str/agi armor threshold (e.g. steel = 24/24, cry = 72/72). However, since the GC str raises faster than GC agi, it is really the agi value that limits these bonuses. Finally, note that there is no 'emerald' bonus at 56/56.
My estimates for GC mods which will give you the bonuses (for each race with GC), as well as the size of the bonuses follow:
str/agi | +AV | +WV | Hara | AHara | Seyan |
10 | 14 | 16 | 1 | 1 | 1 |
12 | 24 | 20 | 28 | 27 | 29 |
16 | 34 | 24 | 37 | 35 | 39 |
24 | 44 | 36 | 55 | 53 | 58 |
40 | 54 | 48 | 92 | 88 | 97 |
72 | 64 | 60 | 165 | 158 | 174 |
90 | 80 | 72 | 207 | 197 | 218 |
I did a little testing to see how accurate these threshold mods are, and while they aren't perfect, they are close (I was able to get 72 agi at 157 mod instead of 158 on an AH, for instance).
So, the take-home is that GCs do not suddenly get stronger each rank. Your best bet is to follow the above table and ID your GC when you start getting close to a threshold.
Hope this helps!
- Thoreusc (Pause, Shift)