After having installed and upgraded to 5.0MP3RP5, it was not possible to configure GAB
# lltconfig -c
# gabconfig -cx
GAB gabconfig ERROR V-15-2-25002 I/O error
Issue appeared to be a duplicate cluster id on the network, as referenced in /etc/llttab.
/etc/llttab was amended to set a unique cluster id of "99" on the test network.
# cat /etc/llttab
link en1 /dev/dlpi/en:1 - ether - -
link en2 /dev/dlpi/en:2 - ether - -
LLT and GAB were restarted, and GAB seeded without error.