Is that relevant when we talk about GDP ? Even if little money flows from Barcelona to the central government (I don't know if this is or is not the case, since I don't live in Spain), that shouldn't make a difference, since "GDP measures the monetary value of final goods and services - that is, those that are bought by the final user - produced in a country in a given period of time (say a quarter or a year)."[1]
Maybe you were thinking about taxes ?
[1] "http://www.imf.org/external/pubs/ft/fandd/basics/gdp.htm"