In the 23rd century, Federation representatives constantly sought to send ambassadors to Gideon in order to get it into the Federation. In 2268, the Gideons finally gave in and allowed representatives to come to their planet. Prior to these representatives arriving, the Gideons submitted reports that made their planet seem like a paradise because the planet was germ-free and the inhabitants had expanded their life spans. These factors had actually caused massive overpopulation on Gideon.
When the USS Enterprise finally arrived, the Gideons only allowed one person, James T. Kirk, to beam down to the planet. The Gideons then set about deceiving Kirk so they could get the Vegan choriomeningitis, for which Kirk was a carrier, and introduce it to their planet. (TOS episode: "The Mark of Gideon")