LiveUSB

Get the ISO image
Good choices are:
 * the architecture appropriate Gentoo LiveDVD
 * SystemRescueCd, which is Gentoo-based. Both 32-bit and 64-bit are supported by the same ISO.

Notice Gentoo's Minimal Installtion CD is not recommendable for Live USB, because it can't be booted from UEFI.

With that being said, almost any modern LiveCD should work for installing Gentoo.

When downloading, adjust the architecture (,, , , etc.) portion of the URL to match the system's CPU. Gentoo mirrors are here.

Convert the ISO image to hybrid mode
Most modern LiveCD's, like Gentoo are already in hybrid mode. If the LiveUSB does not boot, then it will have to be converted to hybrid mode. Hybrid mode means image will enable the ISO to boot from both a CD-ROM device or a USB drive.

Convert the ISO with the following command:

The command comes as part of the  package.

Write the ISO image to a USB drive
Write the ISO image to a USB drive using the utility:

The utility will not return any progress output on standard output (it can provide some progress output, but not without some finagling). The command will exit without any errors (no output) when it completes the transfer successfully. Depending on the size of the ISO image and the speed of the USB device, this process could some time. Be patient!

Once complete, the USB drive should be bootable.

Boot via the system's firmware interface
When attempting to boot from a USB device, be sure to select or enable USB drives as bootable devices in the system's firmware interface. If the ISO is UEFI capable, it may be necessary to enable booting UEFI devices as well. This is typically performed via toggle options in the BIOS or UEFI firmware interface.

While in the system's firmware interface the boot order may need to be adjusted for USB devices to boot first, although it usually easier to hit the appropriate key (usually or ) in order to manually select the USB device as a one-time boot option from the bootable devices list.

External resources

 * System Rescue CD