GHO files are full backups of an entire device, normally a hard drive, created using the now discontinued Norton Ghost program from Symantec. After the 2013 discontinuation of Norton Ghost, GHO files could be created using Symantec Ghost Solution Suite.
GHO files can be opened with Symantec Ghost Solution Suite. For a free program that can open GHO files, use Ghost Explorer, which is a portable program that lets you can extract particular files and folders from the GHO file and save them to a custom destination.
I just googled it in like 5 seconds... Maybe you could do the same next time?