None of the 6 communities that responded have security gates. I have decided to take this inquiry a step further since I am very curious about this question and also interest was indicated among respondents. So I am in the process of contacting all of the communities listed on the cohousing network directory with the same questions. I will post results to this list when they're in. As it's going so far, I've heard back from about 25 additional communities.
Most communities are in more rural settings without crime problems.
No one has yet responded that they have chosen to put up a security gate. East Lake Commons, located in a very transitional Atlanta neighborhood, is a developer-driven community, and does have a security gate. Our developer made the decision to put it up, since it was necessary for him to get construction loans. Several families moved in because they felt the gate would certainly stay, and they wouldn't have moved in otherwise. Now that construction is complete and our HOA has taken control of decision making, other community members want to either get rid of it, or keep it open at least sometimes in the spirit of compromise.