Our team has just moved to TFS 2015 for the start of a new project and I've set up the Kanban board for the backlog items.
What I wanted to ask was whether it makes sense to create a separate taskboard for writing the (gherkin) specifications for our user stories? Is this common practice?
I'd like to track the writing of the specifications for the user stories that have been written and ensure that each specification goes through review by, for example, a QA person.
No, I'd say it's not a common practice. Fundamentally, writing the specs is a task that's part of the acceptance criteria of your story.
Some options:
Put a column in your backlog for "Spec Review" so that you can ensure that the story passes through spec review as part of moving from "committed" to "done". Have a task (or several tasks) for writing the specs and ensuring they pass. If the task is still open, the spec isn't written.
For each user story, make a task for writing the Gherkin specifications. If you're using Git, you can then have your specifications committed to a branch with a branch policy requiring the opening of a pull request to merge the specs into trunk, with a required review prior to the merge being allowed.