FileInfo.com - The File Extensions Resource

.G File Extension

File Type	BRL-CAD Geometry File
File Format	File
Common?	No
File Description	Binary geometry file format used by BRL-CAD, an open source Constructive Solid Geometry (CSG) solid modeling system; saved in a database format that provides a robust and flexible object storage framework.
Program(s) that open g files	Mac / Windows / Unix: BRL-CAD

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/g