GADFONT   FontName  FontSize  MASK(Underline|Bold|Italics)

Gadget text is drawn in the default font, or the font declared by the
WinFont command.

You can, however, have a different font for every gadget.
This is the command that does it.

The MASK is a 3 number string (see the WinType command)
1st Number  : Underline ?      -   1=Yes, 0=NO
2nd Number  : Bold
3rd Number  : Italic

e.g. GadFont times.font 11 010

This defines Bold 11 point times font 

For a font name, it is always best to use the #MONO or #SCREEN keywords.
You can find more on them  Here 

With Dir Listviews specially, you want to use the #MONO font, since
you want the filename etc lined up and a proportional font would look
ugly..