How can I erase my iMac hard drive from a perpetual gray screen?

Big troubles with a troubled iMac. Couple of weeks ago, I got a hang-up on a PSD photo file while scaling. Adobe steps in and says there's a corrupt file, go to cloud to resolve. Wish I could but after progressive hang-ups, including not able to move folders and files, was blessed with the gray screen. In short, about 50 restarts with every possible boot-up configuration, Apple tech, then to Micro Center, brought back an erased drive to restore by Time Mach back-up (Cmd R). Working good, but on the restore when it had 43 minutes left and back to the gray screen. Again, restarts, tried every which way to get a window so I can erase the drive (next time I will restore manually).

Is there anyway I can get to some sort of Utility to erase the drive?