Looks like Debian/Ubuntu and derived Linuxes will not materialise in gui. Testing Debian 10, Ubuntu 19, Kali and Parrot current and no GUI lights up, just a cursor. I am able to get around this by non-GUI grub-bhyve install methods, but that will not work for Parrot.
details: https://lists.freebsd.org/pipermail/freebsd-virtualization/2019-July/007604.html