in the middle
I can confirm that the 1000% GM bonus offer is real, since I've gotten that offer before and I used it to purchase a lot of GMs.
Although, once you've purchased any GM amount or HC in the store, you will no longer get 100% and above GM bonus offers anymore.
(Unless there is an event that offers 100%+ GM bonus offers)
I got that %1000 offer twice also several %700-800 and at every case i was already used store.
I can confirm 1000% is or was a real thing.