This is the song markup that can be used:
All this info is read from the id3tags. %name% -Name of the playing stream. %artist% -Artist name %album% -Album name %title% -Song Title %track%" -Track. (as defined in the tag, this can be a single value but also a string.) "%time%" -Total song time %date% -The date of the song "%file%". -Filename (includes path and extention) %shortfile% -filename withouth path and extention.
The  operator is used to group output such that if no metadata delimeters are found or matched between '[' and ']', then none of the characters between '[' and ']' are output. '&' and '|' are logical operators for and and or. '#' is used to escape characters. Some useful examples for format are: "%shortfile%" and "[[%name% - ][%artist% - ]%title%]|[%shortfile]"
Gmpc also supports breaklines, so you can display the info like:
|The Very Best off (track: 9)\t|
Example markup for this could be: [%name%:
&[%artist% - ]%title%]|%name%|[%artist%
The markup editor in version 0.12 allows you to preview youre markup format. The markup's arent instant apply. In the playlist songs use the new markup when added later, or the id3 info has changed. In the file/artist/search browser the new markup is used when the data in the right pane is changed. In the player the new markup is used after song change.