That's a warning, not an error. It is not stating that the procedure failed, only that some older BIOSes cannot boot from such a device. Since this depends on the computer and not the ISO file, the only way to test it is to dd the file to a USB device an try it.
AFAIK the Gentoo discs are already hybrids, but running isohybrid again shouldn't do any harm.
"Insanity: doing the same thing over and over again and expecting different results." (Albert Einstein)