Gnu grub when installing ubuntu on a usb flash drive

I have a 64 GB USB flash drive that I installed Ubuntu on so I can plug it into any computer and boot to it, but I cant find it in the bios as a boot option. I used an old laptop to install Linux onto the USB and whenever i turn off the laptop there is this thing called gnu grub that is only on the laptop, I assume I need this to boot off the USB so how do i get it to be portable? or how do I boot off the USB with out it?