I found a workaround for my issue. In the planet file Gael A located in ~/GPP/GPP_Planets there is the line:
"removeLaunchSites = Desert_Launch_Site, Desert_Airfield, Desert_GroundObjects, Woomerang_GroundObjects"
Removing this line and thus retaining these vanilla launch sites has resolved the issue. In my install of the mod other bits and bobs have been removed in my attempts to get things working, but this line being removed ended up making the difference, so whether this fix needs to happen in conjunction with other things (I removed the launch sites - beyond the default one - that comes with the mod) I don't know, and I honestly no longer have the patience to test for that.
