Gearrin posted a 3.92 ERA with 39 strikeouts across 41 1/3 innings for Seattle this campaign. The 33-year-old will be a free agent after the season.
New York released right-hander Domingo Acevedo to make room for Gearrin on its 40-man roster.
Seattle, meanwhile, recalled left-hander Justus Sheffield from Double-A Arkansas, the club announced. The Mariners' No. 9-ranked prospect, according to MLB Pipeline, Sheffield is scheduled to make his first career major-league start Friday against the Toronto Blue Jays.
The 23-year-old owns a 4.53 ERA between Double-A and Triple-A in 2019. He's appeared in one game for Seattle this season and gave up two earned runs in three innings.