UPDATE 30/05 3.30PM BST: I tracked down creator Balasz Kaloscai to find out a little more. As suspected (as was obvious), these aren't actual C64 games but his artist's impression of them.
"I simply tap into my memories, draw a massive amount of sprites and backgrounds that roughly evoke the spirit of the C64 and then spend weeks animating the whole thing," Kaloscai told me.
"To entertain not only myself, but younger and older gamers alike," he answered. "And these videos have become the main attraction on my channel ever since the first one, featuring a C64(-ish) rendition of Vice City, exploded all over the web."
Rockstar picked that video up and shared it on its website, hence the explosion - and hence his logical decision to stick with the Grand Theft Auto subject matter.
"I'd like to do at least four-to-five more this year," he added. "Not sure about the games but Blood Dragon and the new Assassin's Creed will definitely be among them."
ORIGINAL STORY 27/05 11AM BST: Didn't Grand Theft Auto 5 have great graphics when it was released in 1990 on Commodore 64?
Oh, what, you don't remember? Well here's a video to remind you.
Oh it's not real really. The video comes from the brain, hands and eyes of Hungarian animator Balazs Kalocsai, and from his '80s-drenched YouTube channel Majami Hiroz.