It would be so nice to be able to view activities by piece of gear. Now, I know that in the web version it's somewhat feasible, but:
There's no such an option in the app, and that shouldn't happen.
You can filter activities on web, but it must be a "double match", e.g., type of activity and piece of gear. In poor words, I can watch all my runs with a certain pair of shoes. But I can't watch multiple type of activities with the same piece of gear, e.g. I can't wiew walks AND runs with a certain pair of shoes.
This is something that can be good value for many athletes who use multiple pieces of gear. I alone have four pair of shoes, two bikes, my brother's bike, and so on...