Gait detection can’t detect a pebble in a shoe, it just detects a different gait. Depending on which side and the location in the shoe, you could have many unique gaits. Nobody will be noticing that you have a pebble in your shoe, and if someone managed to notice that you have different gaits all the time they would probably say something like “oh there’s that guy who always shows up as someone new.” If they notice.

It would be great if a lot of people did this regularly just to throw off the data.