Gaius is the recipient of letter number three from the elder. In it, the elder praises Gaius for all the good reports he's heard about him (3 John 1:3). See, Gaius has welcomed some of the elder's missionary friends into his home (3 John 1:5), and he also "walks in the truth" (3 John 1:3)—huge plus for the elder.
Because Gaius is such a swell pal, the elder cuts his letter short and promises to visit soon so they can "talk together face to face" (3 John 1:14). If he had Facebook, he'd probably be all up in his timeline.