Build a kernel with: genkernel --install --bootloader=grub2 kernel then do a second build with --noclean: genkernel --install --bootloader=grub2 --noclean kernel and watch the whole kernel beeing rebuilt again.
Please read documentation. > ERROR: Unknown option '--noclean'! I assume you meant "--no-clean". But please, read documentation again (see --mrproper option). If you still believe you found something please attach genkernel.log from second run.