Before his senior season, he asked head coach Bill O'Brien if he could convert to offensive tackle since he knew that would improve his chances on playing in the NFL.

[2][3][4] 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.

[8] 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.

[9] 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.

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.

[12] 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.

[13] 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.

[15] 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.

In his spare time, he enjoys working on graphic designs and graduated from Penn State in December 2014 with a triple major in Business, Advertisement, and Psychology.