Hi there. My son was a terrible early waker. The Groclock CAN be amazing but it doesn't work by itself - it will need you and your DH to be 100% consistent in how you use it. I would give your DS a pair of ear defenders (£10 on eBay), a white noise track playing in his room, and instructions to put on the ear defenders and go back to sleep if his sisters wake him. Even if he loses a few nights sleep, he will be better rested in the long run.
Is the stair gate at the stairs, or at they r bedroom door?
My son was younger. Is their receptive language good, i.e. they can understand and follow instructions? We told him, when you see the moon, mummy and daddy are sleeping, and you need to go back to bed and lie down. We then had 3-4 mornings of screaming. I didn't even go to him, but when he would quiet down for a moment I would call out from my room to say "Look at your clock. Mummy is asleep. Go back to bed now." On the 3rd morning he quieted down, on the 4th morning he went back to bed. Then we started to move the clock on by 10 mins every couple of nights until we were at a 7am wake up time. Thanks to the clock he was getting more sleep at 30 months old than he was at 18 months old.