In Australia GCM is Gross Combination Mass, the maximum the total of the vehicle and trailer may weigh. GTM is Gross Trailer Mass, the weight of the trailer minus the towball mass (TBM). From distant memory in Europe and/or UK GTM is Gross Train Mass, equivalent to GCM.
GCM is not necessarily the GVM plus the maximum permitted ATM (total weight of trailer), not least because there are rear axle load limits which must not be exceeded and that would certainly be the case with a very heavy trailer non-steerable trailer and vehicle at GVM.
I have no concerns about using the FL2 hooks and commend LR on being one of the few manufacturers to provide these important safety devices.

