It basically means 'Up Barcelona' or 'Go Barcelona'.
Gora is actually Basque, it's for for 'Up' , not sure why it's used in Catalonia though as they speak Catalan, but that's football for you.
Barna is the slang name for Barcelona, I think it has derivation of a slang term for prison, I'll ask the little lady to confirm that when I speak to her.