You can find the floor plans on
www.vidanta.com. Go to Riviera Maya in Destinations, then Rooms & Suites, then the level (e.g., Grand Mayan), then the room size you're interested in. There should be a link towards the bottom for "show floor plan."
All of those are nice - as you would expect, the furnishings and amenities improve as you go from GM to GB to GL. The size of the units goes up as well. Additionally, if you're staying in GB you have access to the Beach Club; in GL you also have access to the GL pool and Custom Burger Restaurant. The Lazy River is supposed to be restricted to GO as well, but I'm not sure if it's open yet.
There are trade offs for location on the resort grounds, though. My impression is that because Vidanta built the lower tiers first and hadn't thoroughly planned out the upper tiers, the more luxurious ones are less centrally located. It's not too big a deal in RM, though, because the resort isn't as large as NV.