<<Can I buy an external power device of some kind and get the internal USBs to work?>>
If your USB ports are not working...you may be able to install a USB controller card, if your system has an empty PCI slot...and your motherboard has no damage.
Since the Gateway support site doesn't list the specs for your system...I can't say for sure whether your system has an empty PCI slot.
You can open the case and take a look.
If so...then you can simply install a USB controller card, such as those depicted Here.