GS Cap in H2H, Hard or Soft?

Is the GS cap in H2H hard or soft?

It’s a hard cap. As written on the rules page (I. e. II. B.):

B. If enabled, there are a maximum 5 SP slots on a given day. The number of slots will decrease as the team gets closer to the GS cap, down to 0 if a team has no more starts available in a matchup