Gaither helped solidify the Chargers' line during the last five games after he was waived by Kansas City. He replaced Marcus McNeill, who sustained a season-ending neck injury on Nov. 10. McNeill was released on Tuesday. Gaither had been an unrestricted free agent for a day.
Johnson was with Baltimore for nine seasons. He played in a Ravens franchise-record 129 consecutive regular-season games and has started 89 in a row, including playoffs. He had 56 tackles and 2 1/2 sacks last year.
"I'm here to be me. I'm not here to be Vincent," Meachem said. "That's one big step. I like to catch a lot of balls. The last offense I was in, we were pretty much a system. Here in this offense they spread the ball out as well, but they have a lot of guys who make a lot of plays."
Meachem was New Orleans' sixth-leading receiver last year with 40 catches for 620 yards and six touchdowns. Jackson had 60 catches for 1,106 yards and nine TDs last year for San Diego, which missed the playoffs for the second straight season.