The base was populated by a small group of Rebels around the time of the Battle of Tatooine. Rookie One relocated there after the Anchorhead Base was destroyed during that battle in the year 0 BBY.
It was discovered by Imperial Viper probe droids and subsequently attacked by Imperial forces which included All Terrain Armored Transports. Rookie One managed to shoot one walker with his snowspeeder and made his way to the hangar and escape along with the rest of the Rebel fleet as the base was overrun by Stormtroopers and Snowtroopers.
Gamma Base was created for the 1993 LucasArts video game Star Wars: Rebel Assault. The planet on which Gamma Base is located is never identified in the game itself, but both the manual for the Sega CD version and the box for the PC version identify the ice-world as Hoth.