"The GRAY Pre-sale will end when our hard cap is hit (12500 ETH) or at 10:30:00 Pacific time on May 10th (at 17:30:00 UTC)."
So the pre-ICO sale will last up until 10th of May and I see there is no min for purchase.
Hm, but they just might hit their hart cap earlier so it's better to buy it sooner than later cuz you might miss out.