Gsellman was non-tendered by the Mets on Tuesday, making him a free agent, Tim Healey of Newsday reports.
Gsellman has been a significant bullpen piece for New York over the past few years, but he appeared in only 17 games last season while dealing with a torn lat muscle. The 28-year-old had a 3.77 ERA, 1.19 WHIP and 17:7 K:BB across 28.2 innings, and he shouldn't have too much difficulty latching on elsewhere, at least on a minor-league deal.