So, background: I wanted to try out a BSD, specifically TrueOS (link).
I installed it and chose the bsd-bootloader. This was a bad choice, though, because that didn’t work on my GPT partitioned EFI-boot hard drive.
I figured, I could try to boot it with grub, but, my manual configuration tests didn’t work out, so maybe I should try to make grub detect it automatically. For this, the file system needs to be able to be read, and it is using ZFS; thus, I need to be able to read ZFS in gentoo. I found this.
During the install, I got the error that CONFIG_ZLIB_DEFLATE was not set in the kernel. I searched and found this thread. From it, I figured out, I should enable the option
Cryptographic API ---->
Deflate compression algorithm
as a module.
Now, I could emerge zfs and modprobe zfs