Gap refers to the minimum distance the two traces must be away from one another when traces are running parallel to each other for more than the parallelism length specified.
Be sure one or more of the nets are marked as Aggressors in the High Speed rules dialog, just below the Tandem section.
Go to Tools > Verify Design > High Speed > Setup and make sure to check the box for 'Check Parallelism'.
Thanks Bill, toggling on the 'aggressor' box did cause it to finally see any violations I was creating. And now I understand the 'gap' value, thanks