Does GSF explore the outer reaches of the Sentinels universe the way Guardians of the Galaxy explores the non-earth goings-on of the Marvel Universe? Or are they unconnected?
They are connected. In fact, there is a direct connection with the promo Sector from the Kickstater.
It is a VERY far-flung future, but it is the same place.
Cool! I assume Voss isn't still around in it though?
According to not-particularly-wild speculation Voss isn't even around now!
Yeah, Christopher has said he is not around, due to STORY.
When exactly was the moment he stopped being around?
When he was Mistbound by NightMist.
... maybe. According to speculation.
True, but that is the current theory.
He did say it wasn't going to be the last we saw of him...
That he did - but we still have a few sets of SotM coming. (And the potential of a promo villain with WotC. That's my hope, anyway! )
I bet Nightmist banished him to a place that has nastier things than Voss.
I also think when he comes back it will be in Spite like fashion, he won't be his own boss anymore.
Either that or Wager Master frees him as part of his testing the heroes.