Hi Brain,
Attached is test case TC6.DGN,as mention by you, just 4 exterior walls ,Match the floor master elevation to 0/14/28 feet as the height of the wall, and keep Create Storey's Overlap Slabs toggle ON, now it would created the name group and gbXML file in out folder as per BB_gbXML_out path in configuration, you can change this path as per your convenience. Hope this helps you out.
Thanks, Sanjay
Brian Thomas, AIA, LEED AP BD+C
GH2 Architects