It was used a long time ago when gateway wasn't even supporting multi rom and you wrote images on your SD cards using Win32DiskImager. If you had problems when playing your games you could write the GW_Testrom to your SD card. It isn't actually a 3DS rom I think, it's just some data that the gateway could read if you did something (Don't remember what) to start diagnostics tests. Depending of the color of the LED on the back of the gateway, you could know if something in it had failed and you could also know if you wrote the image on your SD card correctly and if the gateway considered the SD to be fast or slow.
I don't think it is useful anymore, I doubt the gateway stills uses it even for diagnostics nowadays since it support file systems and multi roms since years.