The 26-year-old appeared in 14 games for the Seahawks last season, starting 13 at right tackle. He also made 16 starts at right tackle in 2015.
Seattle signed Gilliam as an undrafted free agent out of Penn State in 2014.
With Gilliam out of the picture, the Seahawks will have to rework an already questionable line.
The newly acquired Luke Joeckel figures to start at left tackle, while either George Fant or Oday Aboushi is in line to start at right tackle barring another signing or a selection in the 2017 NFL draft at the end of April.
With Gilliam in the fold, Seattle ranked 25th in the NFL in rushing last season and allowed the sixth-most sacks.
Gilliam will likely battle Trent Brown for San Francisco's starting job at right tackle.
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