So, few weeks ago I've installed a second instance of #GoToSocial for testing purpose.
The primary reason was to test #slurp but finally I didn't tested it yet, lack of time but also because the #mastodon instance from which I want to test the import does not let me export the history of messages & medias 
But ok, it's not a real problem for now, I'm not in a hurry to migrate the mastodon account.
For now what I'm trying to understand is how a #gts instance is federating and give me access to the #fediverse.
The documentation is not clear about this, or I'm dumb, it's possible too 
The thing is I feel a little bit alone here:
- My local timeline is almost empty even federated with ~7700 instances; in fact, there's only what I post myself so totally useless
- My federated timeline is filled with toot of people I'm following; that's totally useless too
It's like hem, in fact the fediverse let you follow stuff people on your instance are following and interacting with.
Currently to discover new accounts and follow topics on which I'm interested to:
- I'm following people, they often retoot stuff I can be interested about and then, I sometime discover new accounts to follow
- I'm following hashtags on third party instances -thanks to @phanpy for this functionality- but it's boring because I can't interact on toot without manual switch to instance...
I've been recommended by @erick (here: https://gts.erick.sh/@erick/statuses/01JN0ZCHN4M5BPWR0FFXJVQYNN) to create a dummy account and follow relay tags/instances (especially https://relay.fedi.buzz) to fill timeline of my instance.
It took me some time to understand how to do that because #GoToSocial doesn't allow like mastodon to use relays.
But basically it's pretty simple: to follow the hashtag, for example #GoToSocial
, just follow @tag-gotosocial
To follow an entire timeline of an instance, for example mastodon.art
, just follow @instance-mastodon.art
Then, federated timeline of my #gts instance start to be filled with a lot of stuff; starting to follow hashtags from my own instance now returns interesting content 
There's still some mystery: not all followed hashtags visible in federated timeline are available when I follow them to my instance (sometime yes, sometimes no...) and I still have tests to realize but I'm quite happy for now, it's a big step ahead about my #SelfHosting instance 