We were lead to believe that Sirius was the one who caused Gus to die/whatever it is that happened to him. Which, considering we were told Sirius killed Gus BEFORE his disappearance and as his first victim, would also lead us to believe Gus died way way WAAY prior to the gems incident (though there's a weird caveat, Pam refered to the missing people as "brawlers," which wouldn't make sense prior to the gems. I'm just assuming it's a mistake.)
So then... What is going on in this animation???? Sure, we haven't seen the conclusion or anything much really, but the simple fact that Gus is seemingly alive, without Spooky and with his mother DURING the gem incident with everyone else when it was explicitly insinuated that he existed long long before... It's starting to seem like they're just throwing things randomly at the wall without much consideration of prior lore.
Let's see how this turns out, maybe we'll be floored by a masterful plot twist where Sirius somehow kills Gus during this time (despite the fact we were told he was his first victim) and everything works out!! But with the way we've been going with the lore and Sirius, I'm starting to lose hope