On the DVLA site the GMA is registered under the make "GORDON MURRAY AUTO", so I assume that having it listed under the "Gordon Murray Automotive" is correct, as they also seem to use that name on their site as a brand, not just as a company name.
About the Gordon Murray Design vehicles, it would be interesting to find a registration (the O56LB is not found on DVLA site) to know if they use that name or if they are also registered as Gordon Murray Automotive. They use a similar model name, but from Wikipedia I would guess that they are a separate brand/make, possibly they are older and they changed from one to the other at some point.