I personally see Girgensons as a good fit on a secondary scoring line playing with 2 skill guys who have great chemistry together and then Zemgus being the hustler on that line. I believe he's had stretches of good play when put with good players, but it seems like he's kind of lost his identity as a player after getting bounced around the lineup, Sabres lacking depth/good players to play with and the team losing a lot.
I don't think he has a lot of untapped potential offensively but as a complimentary piece to combination that works i think there's more potential there.
Sabres definitely have more depth for next season so it'll be interesting to see where he ends up. For example something like Zirgensons - Mittelstadt - Sheary could be interesting as a 2nd/3rd line.
But of course there's always the option to just use him as a 4th liner.