@tom79 I'm not sure what you're asking really. Do you want to know the endpoints for subscribing to a !fediverse user on !GNUsocial? If you do a # lookup you'll get this:
curl -s https://social.umeahackerspace.se/.well-known/webfinger?resource=acct:mmn@social.umeahackerspace.se | json_pp
...
"href" : "https://social.umeahackerspace.se/api/statuses/user_timeline/2.atom",
"rel" : "http://schemas.google.com/g/2010#updates-from",
...

The Atom feed, just like an PubSubHubbub enabled feed, has metadata for the subscription endpoint:

curl -s https://social.umeahackerspace.se/api/statuses/user_timeline/2.atom | grep rel=.hub
<link href="https://social.umeahackerspace.se/main/push/hub" rel="hub"/>

To this hub you do the subscription dance for the feed URI (i.e. the Atom link from before): https://pubsubhubbub.github.io/PubSubHubbub/pubsubhubbub-core-0.4.html

The activitystreams signalling - please note that this is optional, that is something else.