Is it possible to force Grand Theft Auto 1 to run at 1080 resolution?
A game needs to internally support a resolution in order to produce graphics at that resolution—there's no way to force a resolution that's unsupported without recoding parts of the graphics engine itself. This can be done with a lot of skill and code injection, but my searches turn up nothing: it doesn't look like anyone has done a graphics engine enhancer for GTA1.
Get a PS1/PS2 emulator and GTA1, then set emulator resolution to 1080p and run it. You can even scale textures, etc.
I guess there's no way to achieve that with the Steam version.