[5] He started slow in his first season, but began to increase his production after his third game against the University of Northern Colorado, when he had 4 receptions for 82 yards and one touchdown.

At the conclusion of the pre-draft process, Gallup was projected to be a third round pick by NFL draft experts and scouts.

[16]Gallup made his NFL debut in the season-opening 8-16 loss to the Carolina Panthers, catching one pass for nine yards.

[17] In Week 6 against the Jacksonville Jaguars, he made the first start of his career over Deonte Thompson, catching one pass for 27 yards in a 40-7 win.

In Week 7 against the Washington Redskins, he caught his first NFL touchdown on a 49-yard pass from Dak Prescott and a season-best 81 receiving yards in a 17-20 loss.

[18] After the release of Dez Bryant on April 13, 2018, the Cowboys entered the season using a wide receiver-by-committee approach, but the production was not there and the team record was 3-4.

[19] During the season-opener against the New York Giants, Gallup caught seven passes for a career-high 158 yards in the 35–17 victory, including a career-long 62-yard reception.

[22] Three weeks later against the Chicago Bears on Thursday Night Football, he caught six passes for 109 yards and a touchdown in the 31–24 road loss.

It was also the first time in franchise history to produce a 4,000-yard quarterback (Prescott), a 1,000-yard rusher (Ezekiel Elliott) and a 1,000-yard receiver in a single-season.

In Week 17 against the Arizona Cardinals, Gallup suffered a torn left ACL on a 21-yard touchdown catch, and was placed back on injured reserve on January 4, 2022.

[32] On March 13, 2022, the Cowboys took a chance and signed Gallup to a five-year, $62.5 million extension, while he was still recovering from the reconstructive knee surgery he underwent on February 10.

Gallup missed the first three games as he finished the rehab process from a torn ACL suffered nine months earlier.

He started slow with just 8 catches for 86 yards and one touchdown through his first four contests, while CeeDee Lamb solidified his position as the team's number one option at wide receiver.

His lack of production caused him to drop behind the team's top pass-catchers (Lamb, Jake Ferguson, and Brandin Cooks).

In the Wild Card playoff 32–48 loss against the Green Bay Packers, he had his best game of the season with 6 receptions for 103 yards, including a 42-yard catch off a well executed pass route.

On July 23, 2024, before the start of training camp, Gallup announced his retirement from professional football at the age of 28 years old.