At the moment, you’ll have to upload those GPX files into Garmin (or Strava/Runkeeper/MapMyFitness, depending on each service’s capabilities). When each GPX file is created, the service will alert CityStrides of the newly saved activity.
If you decide to use e.g. Strava instead of Garmin, you can connect that account to your existing CityStrides account from the Integrations section of your Settings page. This way, you can have one CityStrides account that contains your full history of activities.
It’s on my to-do list to add direct GPX file uploads to CityStrides ( Manual Upload ), but it’s not close enough to being done for you to wait for that feature.