If it makes anyone feel better; the higher you get in GP, the more GP difference between guilds you may see. I've seen matchups of over 20mil GP difference upwards to almost 30mil
However, the higher you go, GP is not the main factor in if a TW can be won or not as the objective is to "one shot" the whole board anyways, so lots of GP doesn't get used.
Mods will also play a very big role in the outcome of a match.
Having lots of useless GP (ex: many toons level 85 that aren't high gear) will also go against your guild. (same applies to GA)
Word to the wise... unless you plan on maxing out your toon, don't even unlock it
Cheers and good luck to all.