I don't have any first-hand experience with either but was shopping for a 2-person tarp myself recently.
The Grace Duo is quite a bit larger than the GG, and has linelocks on the ridgeline in addition to the pullouts. The GG only has webbing loops on the ridgeline.
The GG is lighter, primarily due to the smaller size. From other people's comments, it sounds like the GG is more of a "normal" 2 person size, whereas the Grace Duo is larger than most other 2-person tarps. I think Ron would make it for you in the GG size at lower weight if you wanted the ridgeline linelocks and bonded seams. The lead time is also a lot longer for the MLD tarp (6 weeks.)