Mkay, so this was actually 2 bugs. One part was giving the attacker an extra buff, the second part was the formula to calculate GT damage removal from attackers was capped at 2, so in my tests the 5000 GT was only removing 2 total damage from the attacker... From the game guide:
Reduce ranged attack by 40 damages per tower (max 2 damages per unit).
So 5k GT could actually negate the attack power of 100k archers (20 archers per tower with the max negation effect, as Human archers have 2 attack each).
I made adjustments, reran the battle:
We managed to kill : 0 Knight 0 Infantry 0 Pikeman 0 Heavy Infantry 0 Archer 0 Mounted Archers
But we lost : 0 Knight 0 Infantry 0 Pikeman 0 Heavy Infantry 7536 Archer 0 Mounted Archers
There it is, the attackers archers were totally negated by the GT. Testing GT more, ran it with only 250 GT, which should knock out around 10k attack power from the attacker:
We managed to kill : 0 Knight 0 Infantry 0 Pikeman 0 Heavy Infantry 2674 Archer 0 Mounted Archers
But we lost : 0 Knight 0 Infantry 0 Pikeman 0 Heavy Infantry 7075 Archer 0 Mounted Archers
Looks good!
Next testing Medical Centers, maxed out ratio:
We managed to kill : 0 Knight 0 Infantry 0 Pikeman 0 Heavy Infantry 7210 Archer 0 Mounted Archers
But we lost : 0 Knight 0 Infantry 0 Pikeman 0 Heavy Infantry 6926 Archer 0 Mounted Archers
That doesn't seem right... Tweaked it to make sure the MC were only counting in the post-battle phase, it seems before they were counted as part of the battle phase - but not sure they were actually doing anything at that point, but they don't need to be there since they don't affect battles (just casualties after the fact). Updated attack stats on a maxed out MC defender look a LOT better with obviously fewer casualties:
We managed to kill : 0 Knight 4715 Infantry 0 Pikeman 0 Heavy Infantry 0 Archer 0 Mounted Archers
But we lost : 0 Knight 6954 Infantry 0 Pikeman 0 Heavy Infantry 0 Archer 0 Mounted Archers
I am confident the building modifiers are working good as intended now... Although I kinda worry about GT being OP? Lol