G'Kar's hatred for the Centauri can be traced back to execution of his father during the 1st occupation of his homeworld. Now, the Centauri have again imprisoned his people on Narn, dissolved their government, and stripped him of his title. After over a year in shackles, Narn has been freed. Upong being offered a position of ruler of his people, G'Kar declined and has returned to the station.
G'Kar recently signed a treaty with, surprisingly enough, Londo Mollari, throwing Narn's official support, along with the Centauri, behind Babylon 5 and The Resistance. It was G'Kar who first supported Sheridan's idea of nullifying the treaties that Earth had made with various League governments, so it is rather fitting that Narn be the first to officially support B5.
Ambassador G'Kar