iMac screen shows nothing after replacing hard drive
Press the Option (or Alt) ⌥ key on boot. This will allow you to enter the Startup Manager and boot from the USB device.
I do not know why the screen is gray and doesn't simply default to the Startup Manager automatically. That would be a nicer experience since the gray screen that never does anything is obviously a bit confusing.
Whatever the reason for the gray screen, explicitly pressing the Option key before the boot chime works. The display was fine all along. The issue was just that the mac wasn't doing anything by default, and launching the boot manager explicitly then booting from the USB installer works fine as expected.