The offer is for $2.2 million, per ESPN's Adam Schefter.
Since Gilliam is a restricted free agent, the Seattle Seahawks have five days to match San Francisco's offer sheet if they want to re-sign him.
Kapadia reported in March the Seahawks offered Gilliam a $1.797 million tender. The salary wasn't guaranteed, and should the 26-year-old sign elsewhere, Seattle won't receive compensation.
While Gilliam started 29 games for the Seahawks over the last two years, the team may not be desperate to re-sign him. The Seattle Times' Bob Condotta wrote Seattle might move Germain Ifedi to right tackle ahead of the upcoming season, giving the Seahawks a potential replacement for Gilliam.
The Seattle offensive line ranked 26th in adjusted line yards and 25th in adjusted sack rate, per Football Outsiders. The fact the Seahawks tendered Gilliam at the lowest level possible wasn't exactly a ringing endorsement of his abilities, so letting him walk could be a part of the team's plans to rebuild the line.
The 49ers' line was poor in its own right, sitting 32nd and 30th respectively in adjusted line yards and adjusted sack rate. San Francisco likely views Gilliam as healthy competition for incumbent starter Trenton Brown.
According to Kapadia, Gilliam turned down a multiyear offer from the 49ers, presumably in the hope he can build his value back up with a strong 2017 season before hitting free agency again.
Considering he'll likely have a chip on his shoulder heading into next year, signing Gilliam could be a sensible, cost-effective move by San Francisco to bolster its depth at right tackle.