On May 9, 2014, in the 2014 NFL Draft, Gillmore was drafted by the Baltimore Ravens in the third round, 99th overall by the Baltimore Ravens. With starting tight end Dennis Pitta suffering a hip injury, Gillmore was expected to play a larger role than first expected. In 15 games Gillmore had 10 receptions for 121 yards for one touchdown on 15 targets. In the Wildcard Round of the playoffs that year, Gillmore recorded only one reception, but he took it 21 yards for a game-sealing touchdown, helping the Ravens defeat their division rival Pittsburgh Steelers in the playoffs for the first time.
2015 season
In the Ravens' third preseason game against the Washington Redskins, Gillmore broke through many defenders to score a long touchdown. The play was called back by penalty, but the media praised Gillmore for putting his determination to make the team on display. At the end of Baltimore's Week 1 game against the Denver Broncos, Gillmore was on the receiving end of a Joe Flacco pass that could've been the game-winning touchdown, but had the ball taken from him by former teammate Darian Stewart, ending his team's chance at a comeback. Gillmore would rebound the following week against the Raiders with a five-catch, 88-yard performance in which he caught two touchdowns, one of which he scored after bulldozing through three defenders. However, the Ravens still lost 37-33. Gillmore caught his third touchdown of the season in a Week 8 win against the San Diego Chargers. He scored for the fourth time two weeks later in a loss to the Jacksonville Jaguars. In Week 11, Gillmore had 100+ yards receiving and led the Ravens in receiving, both first time career accomplishments, catching a career-high 101 yards off of five catches in a 16–13 win over the St. Louis Rams. On December 21, 2015, Gillmore was placed on injured reserve with a back injury, ending his season. Gillmore's 2015 campaign saw him record 33 receptions for 412 yards and 4 touchdowns.
2016 season
Gillmore suffered a thigh injury in Week 7 against the Jets, and was inactive for the rest of the season. He ended the season playing in seven games with five starts, recording 8 catches for 71 yards and one touchdown.
2017 season
During training camp, Gillmore suffered an MCL injury which required surgery, causing him to miss the 2017 season. Gillmore added 40 pounds and attempted to make the switch to offensive line in the 2018 offseason, but was not signed by a team.
