That guy runs "XFCE" on OpenBSD 6.1 on a Ultra 45:
https://www.youtube.com/watch?v=_h9zVCknYPo
And i read with
"XFCE" (
http://daemonforums.org/showthread.php?t=7865,
"xfce4 >> the best x-windows manager !") there are less problems on sparc64 than with gnome...?
So maybe it's easier (no special configuration required?) to install it?