I don't knwo about GSM being easier or harder, I don't have that one leveled. But yes, it should be easier as you level with cross class. Utilizing the lvl 15 skills will make leveling very easy especially if you max out weaver for Careful Syn II. That makes leveling seriously easy.

Lastly don't forget about experience. You know your way around, you know leve's now, mats etc. Bound to increase speed due to that