If your USB drive as it is now (made with Unetbootin) is booting other computers, it should boot the Gazelle too. If it doesn't, I don't think it is Unetbootin's fault.
If your USB drive as it is now (made with Unetbootin) does not boot other computers, try mkusb to flash the iso file to the USB drive.
Some USB drives do not boot some motherboards, while other USB drives will be happy booting that motherboard. So it is worthwhile trying another one or try chainloading. Maybe you can borrow a pendrive or get one that is known as a good booter. See these links
Howto help USB boot drives
Edit: In order to identify the manufacturer and model identification string of all mass storage devices including those connected via USB, you can run this command, which is used by mkusb8-rc2
ls -l /dev/disk/by-id| grep [a-z]$|cut -d ' ' -f 10,12|sort -k2|grep -e \^a -e \^u|sed 's#../..#/dev#'