Since GMS 18 only works on SLES 12, I decided the first step in my overall GW18 upgrade would be to upgrade my GMS server from SLES 11SP4 to SLES12SP2 (with a further step to SP3 after, as per the GMS documentation).
The GMS 18 documentation here: https://www.novell.com/documentation/groupwise18/gwmob18_guide_install/data/inst_req_gw.html says that GMS 18 will work with GroupWise 2014 or later.
Therefore I decided after completing the SLES 12SP2 upgrade, I'd go ahead and install GMS18 so that part of the upgrade would be done. The installation stopped me pretty quick saying that GMS 18 requires Groupwise 18.0.0 or higher. When I answered "No" to the question of whether I had 18.0.0, the installation ended immediately.
I then proceeded to re-install the gms 14.2.2hp2 version, except using the SLES12 iso which I had wisely already transferred to the server just in case. That went fine and GMS is running on SLES 12SP2.
I'm assuming it's the documentation that's wrong, and that the GMS 18 install script has the hard stop for a good reason.
GMS 18 can't run against GW 2014?