Well, then the system could flag you because it doesn't know who you are. Someone walks out of a building with a gait that doesn't match the gait of anyone known to have entered the building. It would follow the person to the next building and the next, until it figures out who they are via the process of elimination (or if it see you go back to your normal gait). Then, the system would note (and store forever) that you have multiple gaits, and would never fall for the trick again.
you have way more confidence in AI than I do. "never fall for the trick again" bwahahaha. it can't even tell the same answer twice, or tell you the correct answer the first time for everything.
This wouldn't have to be AI. I think you are underestimating how good tracking systems already are, and will continue to improve, and become cheaper and cheaper to deploy.