GitLab.com does not allow you to easily export your repositories, wiki's and issues to your own GitLab instance.
Of couse you can git clone the repositories and import then with <ahref="https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/tasks/gitlab/import.rake">a rake task</a>.
Of couse you can git clone the repositories and import them with <ahref="https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/tasks/gitlab/import.rake">a rake task</a>.
And since the wiki is backed by a git repo you can clone that as well.
But these are manual actions and for the issues there is no automation available except for <ahref="http://doc.gitlab.com/ce/api/issues.html">the issue API</a>.
So if you plan on using your own server eventually please consider doing so from the start.