How to get Strava route GTX file in to Garmin Connect as an activity

You can't import a GPX route as an activity because it doesn't contain any timestamps, it's just coordinates.

You would need your friend to export the activity as FIT ("original"), GPX or TCX, then send you their file. If they send you a FIT file, you can strip out any unwanted fields (like HR) with https://www.fitfiletools.com/. In the case of GPX or TCX, you could manually edit the file in a text editor.

https://support.strava.com/hc/en-us/articles/216918437-Exporting-your-Data-and-Bulk-Export

EDIT: or as druidh pointed out, you can take a GPX route and use https://gotoes.org/strava/Add_Timestamps_To_GPX.php to add timestamps, which will turn it into a GPX "activity".

If you can get your friends GPX file then you can add timestamps to it in this utility, 


https://gotoes.org/strava/Add_Timestamps_To_GPX.php


Then upload it to Connect.

Thanks to Druid and FlowState for your helpful replies. Time to have a chat with my friend!