I'm not sure if this will help, but here's what I know.
If you add a new piece of Gear, and set the First Use to a past date (ex: June 1 2015), in the Automatically Add to... section, you should get an additional option that says "Existing Activities ... Add to all XXXXX activities since 06/01/2015" where XXXXX is the type of activity you selected in the section right above.