When Barry, Cisco, and Caitlin first finds Gideon, she tells them about Barry saying "Founding member of..." but is interrupted by Barry. What exactly will Barry be the founding member of?
Most probably the Justice League.
From the preview comic for Episode 20 of Season 1 of the Flash, The Trap on the CW's twitter, they reveal that Gideon was cut-off after she said he was the" Founding member of the Justice..."
Could be Justice Society of America...Because Batman is the founding member of Justice League