They might only have one GPU each but there are a lot more of them, if even half of all people who bought RDR2 complete it that would be similar to 1.5 billion GPU hours of crypto mining (Eth for example).

32 million bought it, takes about 100 hours to complete runs GPU full capacity.

If we're going to say using GPU computing power is bad for the environment then we can't just stop at crypto because it's convenient. We have to look at all computing and videogames isn't something that should be considered essential either.