These fonts are the equivalent of the following standard 
Windows fonts:

fixed.dfn       Fixedsys
fixed12.dfn     Fixedsys (12pt)
system.dfn      System
system12.dfn    System (12pt)
large.dfn       MS Sans Serif (18pt)
small7.dfn      Small Fonts (7pt)
small8.dfn      Small Fonts (8pt)
small10.dfn     Small Fonts (10pt)

The -gdifonts option forces Doomsday to use the original
GDI fonts instead. The -fontdir option can be used to 
specify a different DFN directory (default is "fonts\").