Enabling App Links

If you want to open Reddit links in the app, you need to enable Geddit as the default app for Reddit links.
To do this go to:

Settings > Apps > Default apps > Opening links > Geddit

Then select Add Link and enable all the available links.

I can't enable any links!

Only one app can be the default app for the same app links.
Therefore, you need to deselect the official Reddit app as the default app for Reddit links.
To do this go to:

Settings > Apps > Default apps > Opening links > Reddit

Then disable Open supported links.
Now you can select Geddit as the default app for Reddit links.