tetris.img is a floppy disk image. You can use e.g. rawrite
to write it onto a floppy disk, and then you reboot the
machine with it.

Or, you can "mount" it and use directly. In Linux:
  mkdir newdir
  mount tetris.img -o loop newdir
  cd newdir
  dosbox
  mount c .
  c:
  gwbasic tetris

Or, you can use a virtual machine / PC emulator
to "reboot" without actually rebooting. In Linux:
   kvm -fda tetris.img -boot a
or:
   qemu-system-i386 -fda tetris.img -boot a
or:
   dosbox tetris.img


Note: I have experienced problem getting gwbasic react to
keyboard input when running under qemu. Your mileage may vary.