It does escalate. I seem to remember the first 3 GG need 30, 60, 90 XP...
It means what you think.
It's the same amount of XP your unit gained in thet combat (with modifiers if you are imperialistic/Great Wall).
I dont know the exact Formula for combat XP. It depends on how 'hard' the combat was.
I.G. High odds combat will give you fewer XP. Winning at low odds gives you more.
For the most time when attacking you will get 2 XP, unless at really high odds, where you get the minimum of 1. Very risky low odds attacks will give you more XP sometimes - if your unit can win against those odds that is. I think defending allways gives you less XP than attacking. A Withdrawing unit allways gets 1 XP.