A .gmb is a GoldMine Backup file, and it must be restored as a GoldMine database before any program can convert it to ACT! Said another way, no conversion program understands a .gmb.
You'll have to use GoldMine (same version as was used to create the backup) to restore that.gmb to the original Contact Set and GMBase files first.
MigrateAdmin from ASDS wants the ~17 Contact Set files plus some GMBase files in order to convert. The Contact Set files look like:
CONTACT1.DBF CONTACT1.DBT CONTACT1.MDX
CONTACT2.DBF CONTACT2.DBT CONTACT2.MDX
CONTSUPP.DBF CONTSUPP.DBT CONTSUPP.MDX
CONTGRPS.DBF CONTGRPS.DBT CONTGRPS.MDX
CONTHIST.DBF CONTHIST.DBT CONTHIST.MDX
and others
In GMBase (or in the root of GoldMine), you'll find:
