If you are using Windows then the houdini.env file will be in the Windows documents folder. Open it with the notepad editor and underneath all of text that's already in there paste this code.
HOUDINI_OCL_VENDOR = NVIDIA Corporation
HOUDINI_OCL_DEVICENUMBER = 0
Then save the env file and start up Houdini again. When Houdini starts click the ‘help’ tab. Then at the bottom of the list choose ‘About Houdini’ and in the window that pops up put a tick in ‘Show details’ check box. Scroll about half way down to where it says ‘OpenCl Platform’ and it should now have your GPU listed.
-1 refers to accessing the iteration via the first spare input. So you add a spare input via the gear icon that you use to set parameters and stuff like that. You can add more than one spare input and access them with -2, -3, -4 etc.