He was presumably tasked with planting a package of explosives in an area that would be used against the Drone that was terrorizing the station's inhabitants.
Waits contacted deputy Ricardo and informed him that Gardiner was not "checking in". He sent Ricardo to his last known location to evaluate the situation. Ricardo eventually replaced Gardiner's role in taking the explosives and planting them in the area.
Gardiner's presumed corpse can be found near the explosives. It is unknown how he died, though he was most likely another victim of the Xenomorph.