As I understand things, these "gpdks" are foundry non-
specific and so may either lack layertable to mask files,
or these may have to be "bolted on" from a foundry
specific source.
First question is, does the default layertable have GDS
layers or only display layers? If it has GDS info then
check the box saying to use techfile, layertable or
whatever and not a layers file.
You might grep around the PDK root for "DATATYPE"
and "datatype", "LAYER" and "layer", and see what it
turns up. Then maybe you use the popup to browse
to that.