From Halopedia, the Halo wiki

GASO rank icon
The GASO rank as seen on Halo Waypoint.

GASO ("Gold All Skulls On") is a Halo: Spartan Assault exclusive ranking that can be earned by getting a Gold ranking with the Thunderstorm and Black Eye Skulls on. This challenge was only visible on the Halo Waypoint service record for Halo: Spartan Assault.