Whenever Goma Fada Vanguard attacks, target creature an opponent controls with power less than or equal to the number of Warriors you control can't block this turn.
The power of the target creature and the number of Warriors you control are checked only as Goma Fada Vanguard’s ability is put onto the stack and as that ability resolves. If the creature’s power becomes greater after the ability has resolved, it still can’t block.
The target creature is restricted from blocking anything, not just Goma Fada Vanguard.