While GTA VI, according to most rumors and leaks, will be released only in two to three years, Rockstar Games, apparently, is engaged not only in improving GTA V for the PlayStation 5 but also remasters of older parts of the franchise.
According to the source, games may be released this fall, and for multiple platforms, including even Nintendo Switch and smartphones!
The source talks about the remasters of GTA III, Vice City, and San Andreas. All three projects are based on the Unreal Engine and will be "a mixture of old and new graphics", whatever that means.
The game should be expected in late October or early November. They will be released on PC, PS4, PS5, Xbox Series, Xbox One, Switch, Google Stadia, and smartphones. However, it is not necessary that the remasters will be released on all platforms at the same time. There is a possibility that ports for PCs and smartphones will be delayed until 2022.