Johnson, 61, went 130-74 with a 3.42 ERA across 10 seasons with the Mariners.
The Sounders’ starters posed for the pregame photo wearing the T-shirts, which also said “Fair Share Now” on the back.
Following an off day, Mariners RHP George Kirby (0-2, 11.42) gets the ball Tuesday to begin a three-game series against Baltimore.
Mariners right-hander Bryan Woo (5-2, 2.69) gets the ball for Seattle to begin a three-game home set against Minnesota.
Gore (2-5, 3.47 ERA) starts Thursday against Mariners RHP Emerson Hancock (2-2, 5.95) to close the three-game series.
Trevor Williams (2-5, 6.39 ERA) is scheduled to take the mound Wednesday for the Nationals against Seattle’s (George Kirby 0-1, 12.27).
Seattle gets a visit from Washington on Tuesday night.
Houston RHP Ryan Gusto (3-2, 4.85 ERA) opposes RHP Emerson Hancock (1-2, 6.21) when the series continues Friday night.
Mariners RHP George Kirby (no record) comes off the IL for his season debut against Astros RHP Lance McCullers Jr. (0-1, 7.88) at Houston on Thursday.
Mariners right-hander Logan Evans (2-1, 2.57 ERA) and White Sox right-hander Shane Smith (1-3, 2.05 ERA) pitch on Wednesday in the series finale.