LiveUSB

First you need to download LiveCD iso image. Gentoo LiveCD or Gentoo based SystemRescueCd are good choices for installing Gentoo. SystemRescueCd supports both 32bit and 64bit processors on the same image. Almost any modern LiveCD should work.

Get the LiveCD image
You need to change the architecture – x86, amd64, arm, sparc, etc. – to match your CPU.

Convert LiveCD image to hybrid mode
Most modern LiveCD's, like Gentoo are already in hybrid mode. If your LiveUSB doesn't boot then you have to convert it to hybrid mode. Hybrid mode means image that can be booted from either CD-ROM or USB drive.

Write the LiveCD image to the USB drive
Now you can boot. You must select the USB drive as startup device in BIOS.

Other resources

 * Gentoo LiveUSB HOWTO