Ok so I may be stupid. But I didn't actually catch the 'memory loss is a possible side effect' bit. Isn't it the side effect of BRYONY'S treatment? It's discussed in The Boat and the Bridger iirc. Sure, it has everything to do with Warren's PTSD but it's Bryony who was erasing his memory with meds. It's really convenient how he only forgot the criminal part of his life.
Not to mention that Warren didn't experience huge memory loss when he was brought back to Red Valley and he had A LOT of hypersleep. And I get that the technology probably improved with time but none of the people who experienced hypersleep in s3 had any problem with memory.
So yeah. I'm not sure about Gordon losing his memory. At least I don't think it makes sense with all the information at hand.
The only possible reason for Gordon to lose his memory in my opinion is the fact that he was given the degraded goo (sorry, can't for the life of me remember what's it called). But well, I'm no expert. I guess we'll wait and we'll see.