I use a Wahoo not a Garmin currently, and even with Garmin Connect I don’t know that there’s a way to auto-detect bikes based on which ANT+ sensors they use.
My solution to this issue was ActivityFix.com (thanks @WindWarrior!) This is actually a pretty handy tool: it lets you set a series of rules to modify Strava activities based on tons of attributes, including which Ant+ sensors are used. You can easily upload past .fit files to identify and save your Ant+ sensors, then set a rule that (e.g.) if an activity used PM X, change “gear” to bike Y. Strava tracks total miles on each bike, so problem solved. Exactly what I was looking for.