It definitely is another barrier to interaction, but it doesn’t make it impossible. In fact, it could be argued that it will get you participants that truly want interaction and guidance in the community and they could end up being very active users. So don’t lose motivation! This just may be a bigger learning experience, and there’s a lot of value in that. I run a support community that is primarily accessed through a support portal, though it’s not gated. It’s also the community for a community platform whose primary use case is support communities.
Like Hawk said, getting them to stay is going to be extra work. When a community is tied to a support portal, I think it can increase the amount of transactional visits. My suggestions would be to add lots of extra value around support topics and to ask a lot of questions. I think making them feel valued is key. I could go on and on with ideas here, but am worried my response would be too long. Feel free to message me if you would like!