I’ve mentioned this feature in the Wiki post Where are my older Garmin activities? so I should add the details directly here as well.
If you’re looking to upload older activities that Garmin blocks access to, see the linked post above - it has more details that you’ll need that I won’t discuss here.
The Manual Upload page is available to all Supporters now. Visit the page to drag & drop your GPX/FIT files for upload. There’s a frequently asked questions section at the bottom of the page that should answer all your questions, but if you’re wondering anything please leave a comment so I can update the page as needed.
I’d love to hear from anyone who uses this, to confirm that things are working as expected.