…aunque la huelga no iba a afectar la operativa, ya que los servicios mínimos fijados por el Ministerio de Fomento han sido del 100%
This is from an article about the last strikes by this group, December 22 and 27, 2015. My spanish-fluent wife translates this as, at that time, the strike didn't affect service because the government mandated that minimum services must be maintained, and at no less than 100%. So far, for this strike, there has been no such statement.
Could the regular air traffic controllers assume the duties of the ground control staff? As I understand it, at MAD, they used to do it some years ago.