Gus was a former Warehouse 13 agent who work at the Warehouse during the 1960s and possibly the 1970s.
First mentioned in the episode "Claudia" after Mrs. Fredric briefly talked about how she could have saved Gus but didn't for some unknown reason.
"And you might have prevented it all... and I might have saved Gus, but I didn't." - Mrs. Frederic.
Mentioned again during the episode "Age Before Beauty" when Artie remembered that Gus used to talk about an artifact that dealt with photography and could possibly be the culprit for the advanced aging.
Mentioned in "Where and When" when Charlie, the custodian of Warehouse 13 during the 1960s, reassigned Jack and Rebecca from the glass girl case to go to Calcutta as Gus had a lead on Man Ray's Camera's whereabouts.