After installing TrueOS to unallocated disk space, my computer didn't boot properly any more. With choosing a boot image, my screen froze. So I ran the rescue mode of a debian install medium and reinstalled the MBR (Master Boot Record). Now, GRUB acts as if debian was the only system on the drive.
I tried updating GRUB from GNU/Linux, which gave me the following:
$ sudo LC_ALL=C update-grub
Generating grub configuration file ...
Found background image: .background_cache.png
Found linux image: /boot/vmlinuz-4.19.0-2-amd64
Found initrd image: /boot/initrd.img-4.19.0-2-amd64
Found linux image: /boot/vmlinuz-4.19.0-1-amd64
Found initrd image: /boot/initrd.img-4.19.0-1-amd64
Found memdisk: /memdisk
Imagepath /boot/images not found
Now I'm at the point where I take any *BSD which has a GUI by default. How do I have to modify the file telling GRUB which OSes there are available? Which file is it (in)?