You are awarded based on how well you held your ground and performed during the mission (i.e. how well you protected the objective or how quickly you defeated a boss). The highest-ranking Gold Medals are earned if no less than 80% of the primary objective's health remains by the end.

For boss levels, you must defeat the following within the estimated time frame.

Africa: 135 seconds.
Europe: 40 seconds.
Pacific: 320 seconds.