Pre-draft With only a single year of experience at offensive tackle on the collegiate level, Gilliam was not projected to be drafted and was not invited to the NFL Combine. During Penn State's Pro Day, the Seattle Seahawks were one of the few teams to give him a workout. The Seahawks were forward with their intentions on giving him an opportunity and had constant communication with him up until the draft. They also spoke about using him as a tight end in the future if needed. The Seahawks told him before the draft that they would sign him as an undrafted free agent. On May 11, 2014, the Seahawks signed Gilliam to a three-year, $1.54 million undrafted free agent contract that includes a signing bonus of $12,000. Gilliam made his NFL debut at offensive tackle on September 4, 2014, against the
Green Bay Packers. On October 5, 2014, he entered at left guard during the second half of the Seahawks' game against the
Oakland Raiders after
James Carpenter suffered an ankle injury. On January 18, 2015, Gilliam caught a 19-yard touchdown pass on a fake field goal attempt from punter
Jon Ryan during the Seahawks' 28–22 overtime
NFC Championship Game victory over the
Green Bay Packers. It was his first career reception and touchdown.
2015 season Gilliam began his second season as the backup left tackle behind
Russell Okung. After starting right tackle
Justin Britt was unable to perform well during the pre-season, Gilliam was moved into the starting position and earned it two weeks before the season opener against the
St. Louis Rams. Britt was ultimately switched to left guard, with former left guard
Alvin Bailey taking Gilliam's former position at backup left tackle. Throughout the first five weeks of the season he was the 74th-ranked offensive tackle in pass blocking and ranked 66th and run blocking. From weeks 5 to 10, he was ranked 75th in pass protection and 61st in run protection. He was ranked among the lowest throughout then season. On March 14, 2016, he was awarded the fourth-largest amount of additional salary in the NFL's "performance-based pay" program receiving $329,384. The program pays players who play significant playing time and aren't compensated enough due to their contract. Gilliam received the fourth-largest amount since he started the entire season at right tackle, playing 98.05% of the Seahawks' offensive snaps, and was still being paid under his undrafted free-agent contract. His total salary was $839,384, instead of the $510,000 he was scheduled to earn.
2016 season Gilliam was slated to begin the season as the starting left tackle, after
Russell Okung departed for the
Denver Broncos via free agency in the off-season. With the Seahawks installing new starting offensive linemen at every position, Gilliam was moved to right tackle after free agent
J'Marcus Webb twisted his knee in training camp. Free agent
Bradley Sowell was then moved to the left tackle position since it was thought that Gilliam was better suited at right tackle than Sowell. On November 16, 2016, Seahawks' head coach
Pete Carroll announced that there would be an open competition between Gilliam and Bradley Sowell for the starting right tackle position. Sowell had lost his starting left tackle position to undrafted rookie
George Fant after he suffered a knee sprain that kept him out of four games. Two days later, Carroll announced that Gilliam will remain the starter against the
Philadelphia Eagles. On December 4, 2016, it was reported that Gilliam, who had started the previous 27 games, had lost his starting right tackle position to Sowell during the week and would be a healthy scratch for the following game against the
Carolina Panthers. On December 15, 2016, the Seahawks benched Sowell in the fourth quarter of a 24–3 victory over the
Los Angeles Rams and had Gilliam enter the game in his place. He remained the starting right tackle for the last two games of the season and finished his last season with the team appearing in 14 games with 13 starts.
San Francisco 49ers On April 17, 2017, the
San Francisco 49ers signed Gilliam to a one-year, $2.2 million offer sheet. The following day, the Seahawks declined to match the offer, officially making Gilliam a member of the 49ers. He played in eight games in 2017 before suffering a knee injury and was placed on injured reserve on November 7. On February 27, 2018, the 49ers signed Gillam to a two-year, $8 million contract extension with nearly $4 million guaranteed. On February 13, 2019, Gilliam was released by the 49ers. ==Personal life==