feat: add gear support for GC

[florianpasteur]

I created 4 new methods

• getGear to retrieve all gear of the user
• getGearsForActivity to retrieve the gear assigned to a single activity
• linkGearToActivity and unlinkGearFromActivity to assign/deassgin a gear to an activity

I'm also preparing the creation and editing of a gear, but it will come in a different PR.