In case you didn't know, both gaiden series are split into different story arcs that jump to different times, focusing almost exclusively on Reinhart's career. Central Anime has subbed the first 3 arcs of the 1st gaiden, which are all 4 episodes each. The final arc is 12 episodes and hasn't been subbed yet. None of the second gaiden has been subbed by Central Anime yet, but at least one of the arcs deals with Yang.
I'm waiting on Central Anime's subs so I haven't seen everything yet- only the main series, the first movie, and the gaiden so far.
The first movie is awesome though and one of my favorite pieces in the series.