The command Open Folder (menu: File | Open Folder...) is meant to be a general purpose directory importer for "any file types" (like PDF, DXF, GDSII, BOM, etc.) -- and is not specifically optimized for one format, like the other importers in FAB 3000.
If you are loading a large number of Gerber/Drill files from a folder, we recommend using AutoLoad (menu: File | Import | Auto Load Gerber/Drill...). Why? "AutoLoad (Gerber/Drill)" is specifically build for loading in larger numbers of Gerber/Drill files contained in a folder - has specific import format settings just for Gerber/NC.