Geogrid-Viewer overlay files seems to be pretty popular in Austria and Germany. So far, gpsbabel could only read Geogrid track logs and ASCII overlay files, but no binary overlay files.
Here comes a reader implementation for gpsbabel. It supports version 2, 3 and 4.
There is no documentation for the file format, so this was implemented by looking at hexdumps of many sample files found on the internet and two free software packages that include support for version 2 and 3: top2gps, bbbike.