For those who are looking for it

There are two versions of this game: SNES and DOS (Beta channel)
At this time the SNES version will be installed by default

If you want to use the SNES rom file in an external emulator, you will find it here:
[GOG Library Path]\Street Racer\res\game

If your emulator expects a file extension, try to rename the file

For usage in Recalbox 8.1 it was sufficient to rename it to game.sfc