You could start by posting the grub.cfg entry for your HBSD system, grub-install
has never really worked with anything but Linux and Windows, and always required making a custom entry ๐, which will vary depending on your partition layout, root file system, encryption, etc.. non-encrypted ufs2 is usually easy to get booting with Grub or SysLinux on FreeBSD, hope this applies to HBSD too.
Also, gptboot(8) is meant for BIOS, not UEFI ๐