Hey,
I am very experienced with active-admin and have developed various interfaces with it, utilising features such as omniauth integration, custom pages, filters, custom actions, different namespaces, and so on.
I develop primarily on ruby now because of its natural language like syntax and manage all my projects with git. I have worked extensively on gems and rails applications, and I am versed in rails features such as Single Table Inheritance, Polymorphism, Self-referential associations, and so on.
For an example, at the moment, I am developing a gem which uses nokogiri and converts any text markup language (markdown, textile, html, org-mode, etc.) to another one.
Since, I have not taken any previous work in ruby on freelancer, and I am trying to develop a ruby based profile here, I am willing to offer you a trial demo of my skills with activeadmin and git (which are your primary requirements).
Once you are satisfied with my skills, you can consider me for your project, with more trust. Moreover, if you will read my reviews, you will notice that I deliver customer satisfaction and quality of work.