Hi, I had this problem as well. Had completed the goal a month in advance and when I went back to real time the gates would not open , nor the houses be built. So , I went forward in time again the gates opened and I was able to build my first beach house.
One snag was that I then had to move forward again from that point to complete the build. In real time the gates are shut again and I cant build any more. But the house is there. I imagine the others will be buildable in a month when times catch up.
Hope this helps