OMG! I thought 'GPU users' was PCs with GPUs (cloud gaming working on PC's with GPUs)!! This conversation is so confusing. Can we please use GP-U?
And still no-one actually pointed to the statement from CMA that's being disputed.
Okay, investigating, doing the whole job of discussion myself once again, the issue is the CMA asserted MS's current Cloud Gaming share as substantial (70%?) by using Game Pass Ultimate subs as a count of cloud gamers. Someone having a GP-U sub doesn't mean they are cloud gaming. The argument at present is comparing different types of subscription and service, none of which is identical or objectively comparable, and passing judgement on the subjective comparisons.
I'd say at this point the definion the CMA are using is services with a target audience, whether people use it or not. On PS, that's PS+ Premium. Not every PS+ Premium subscriber is going to be cloud gaming, but in the absence of other metrics, I guess that's all we and the CMA have got*. Counting that, the equivalent for XB is GP-U subscribers. For Amazon, Prime Gaming doesn't include anything but a tiny taster, so it's unrealistic to count anything other than Luna subscribers.
* If that's their numbers, and their source, counting these subs. Without reference to the source, I'm filling in the blanks with internet chatter.