Can I react to the Grand Plaza checking each turn if there are 7 units holding it, even if there aren't?

No, you cannot react to The Grand Plaza's check.

According to RiftJudge FAQ #10151 and Rule 383.3.e, The Grand Plaza's ability is a triggered ability that only enters the chain if you have 7 or more units present at the battlefield at the exact moment you perform the "hold" action during your Beginning Phase.

If you have 6 or fewer units at the battlefield when you hold, the condition is not met, the ability does not trigger, and it is never placed on the chain. Because no item is added to the chain, no priority window is created for players to react or add units to the battlefield.