For how long is Gold status granted?

It is recalculated for the first time on December 31st following your transition to Gold status, then every December 31st.
To maintain your Gold status, you must have made 4 flights, i.e., 2 return flights between January 1 and December 31.
If the required number of flights is not been attained, you will no longer be able to benefit from the additional benefits of Gold status but those of Club status.