G'Kar was no mere ambassador: he was a member of the Third Circle of the Kha'Ri, one of the twelve most important Ambassadors of the Narn. He proved himself in this role to a high degree, showing great effectiveness in leading the Narn resistance against the Centauri, to such an extent that he was specifically targeted for removal in any way possible.
The Centauri went out of their way to call him Citizen G'Kar to denigrate him, to emphasise that he was not important, and to remove him from the position of power and leadership he still retained. Their intense efforts did not go unnoticed by the rest of the Narn resistance, however. The Narn took his non-title of Citizen and turned it into a celebration; this was the only Narn singled out for the Citizen treatment by the Centauri, and therefore Citizen G'Kar was the most important Narn in the galaxy.