I am not a GEOM expert, but isn't it wrong concept, that glabel writes its metadata and publish original device size? If some GEOM write metadata at last sector (or first), then it should shrink the published size (or offset). Or is the problem at geom_part, that it is writing metadata past the advertised end of the device?

e.g. If I have disk device with size of 100 sectors and glabel metadata is stored at the last sector, then glabel should shrink the advertised size to 99 sectors - then GPT secondary table will be at sector 99 instead of 100.

The current state is simply wrong, because user can do something what cannot work and is not documented anywhere.