The free-agent receiver intends to sign with the Tampa Bay Buccaneers on a three-year, $30 million contract that includes $20 million guaranteed, a source told ESPN's Jeremy Fowler.
The Buccaneers inherit the remaining two years of Mason's contract, which calls for Mason to earn a base salary of $6.5 million in 2022 and $7.5 million in 2023.
With no Antonio Brown, the Bucs had a drop-off at the No. 3 receiver spot last season behind Pro Bowlers Mike Evans and Chris Godwin, which became apparent after Brown's dismissal from the team and Godwin's season-ending knee injury. Tyler Johnson did not flash and had some struggles at times when taking on a bigger workload.
Gage had back-to-back seasons of 700-plus yards for the Atlanta Falcons in 2020 and 2021 as his role increased following injuries to Julio Jones in 2020 and Calvin Ridley missing 12 games last season to focus on his mental health.
After suffering an ankle injury in Week 2, Gage, 26, missed three games and, even when he returned, wasn't fully healthy until the second half of the season. He emerged over the final two months, catching 11 passes for 130 yards against the Buccaneers on Dec. 5. He also had nine receptions for 126 yards and a touchdown in the season finale against the New Orleans Saints.
Gage set career bests in yards per reception (11.7), catch percentage (70.2%) and yards per target (8.2) last season.
The 6-foot, 184-pounder made 193 catches on 287 targets for 2,065 yards and nine touchdowns in his four seasons with the Falcons after being drafted in the sixth round in 2018.
Mason, 28, reunites with Brady after having spent the first seven years of his career with the Patriots. Mason has played in 103 career regular-season games (98 starts) and 13 playoff games (all starts).
His void will be significant on the Patriots' offensive line, as the team already lost starting left guard Ted Karras in free agency to the Bengals and starting right tackle Trent Brown remains a free agent.
In dealing Mason, the Patriots fill a void on their draft board (they didn't have a fifth-rounder) and up their total of picks to seven. But perhaps more importantly, they pick up $6.3 million in salary-cap space. The Patriots have been tighter to the cap this offseason, so the space will help them.
Mason joined the Patriots as a fourth-round pick out of Georgia Tech in 2015. He played in a run-based offense in college, but developed into a reliable pass-protector in the NFL as well.
ESPN's Mike Reiss, Jenna Laine and Michael Rothstein contributed to this report.