Which letter cluster will replace the question mark (?) to complete the given series?

EDIC, FGJF, ?, HMLL, IPMO

GJKI

Explanation:

EDIC, FGJF, ?, HMLL, IPMO

The logic followed here is :

E → (+7) → F → (+7) → G → (+7) → H → (+7) → I

D → (-8) → G → (-8) → J → (-8) → M → (-8) → P

I → (+9) → J → (+9) → K → (+9) → L → (+9) → M

C → (-10) → F → (-10) → I → (-10) → L → (-10) → O

So, the letter-cluster that replaces the (?) to complete the series is "GJKI"