Its like this
Use GX if you want to finish OGH fast but its expensive since you need to use high end gears. Clearing it will still be hard even if you have the right equipment since you still need technical skills.
Use RG if you want to farm coagulated spells efficiently with mid range gears but its gonna take a while to kill amdarais. Clearing should be easy since you only need to use sac against raydrics then GC the rest of the mobs.
My average time with RG is 40mins