Heh, by the time you're over lvl30, money flows like water. I'm at a point now where i'm CONSTANTLY needing to just buy Gems from the shop as i keep hitting the GP cap (999,999 is max) too often. After a certain point, the only use you'll have for GP are 2 things; 1) buying Gems for upgrading necklaces and feeding pets 2) buying pet attacks.
That is all. GP really loses it's use after a while. Part of why they're making necklace upgrading so damn hard to +9 is there'd be NO use for GP otherwise...