Sabayon uses Dracut as initramfs generator, for further customizations you can refer to the dracut wiki.
Sabayon ships an helper tool to aid user to regenerate initramfs,
You can use it to regenerate initramfs against specific kernel, or for every single one installed in your system.
sabayon-dracut, if not already present in your system, run as root:
equo install sys-kernel/sabayon-dracut
Now if you would like to rebuild the initramfs, for let's say your kernel 4.18, you can just run:
sabayon-dracut --rebuild 4.18
To rebuild the initramfs for all the kernel installed in your system:
You can also check the initramfs present in your system with: