And you changed the boot order to USB first? or is there a one-time boot menu key like F12?

If you did either/both of those and it still didn't boot, then the iso probably wasn't installed onto the flash drive properly. Did you create an iso image of your Vista DVD before loading it to the flash drive? Or did you try the Win10 iso again?

Does your old Gateway specifically say USB in the boot options? It's not USB-HDD or USB-Floppy, right?

You could also try booting the Win10 discs in your other computer. If they boot properly, then it's something to do with your Gateway.