Accomplish the "11th Clan of the S'pht" achievement to unlock the Marathon logo Gamerpic.
Accomplish the the "VidMaster" achievement to unlock the Vidmaster Gamerpic.
Hold the Command + Option buttons and select "Begin New Game". Then, type the level number and select. This works on all Marathon maps, and Marathon 2 and Marathon Infinity as well.
Accomplish the indicated achievement to get the corresponding number of Gamerscore points: