Gustavsson may have cleared waivers during training camp. If not for the budget, they probably could have kept him and sent something else to Minnesota for Talbot. Given how we were likely the only destination for Talbot, I doubt Gustavsson held a lot of value to them. He was likely a throw in that saved them from having to find a Hellberg or PTO type to be their backup.
He was on a 1-way deal, they were still a budget team. They weren't really in a position to gamble on him as their starter as they were trying to polish up the team for a sale, and they needed to make progress with winning games. Of course, that didn't happen, and Talbot was bad. The trade at least made sense in execution.
Minnesota has been bad, and both goalies have abysmal stats. I would not write off Gustavsson based on this bad start. It seems like more of a team-related issue.
On the topic of the Brassard trade, Gustavsson was a pretty big piece. We're talking about a goalie who was selected in the second of the draft who saw his value go up. The only reason he was available was probably because at the time Pittsburgh were stacked in net with Murray, Jarry, and DeSmith. While the pieces from the Brassard trade did not pan out for the Senators, it was a very strong return to get both Gustavsson and a 1st.