The club appointed Robert Brown as manager, but the arrangement turned out to be only a casual one and he accepted another job before the season started.

He was one of three players who tied for the most appearances made during the season: Hall, Jack Branfield and Jock Robertson each missed only one game.

The highest attendance recorded at the club's home ground, Priestfield Road, was approximately 12,000 for league games against Southampton on 28 August and Millwall on 30 October.

[4] Initially it was unclear if Gillingham, by virtue of their last-place finish, would be relegated to the Southern League Division Two before this took effect and thus miss out on a place in the Football League; at the club's own AGM on 3 June angry supporters demanded to know what the club's status would be for the coming season but the board of directors was unable to give an answer.

[10] Only 6 out of the 39 players who had represented the club in the final Southern League season went on to make appearances in the Football League: Jock Robertson, Jack Branfield, Syd Gore, Joseph Griffiths, Donald McCormick and Arthur Wood, and of these only Robertson, Branfield and Wood remained regular starters.

[12] The club's first Football League match was against Southampton at Gillingham's home ground, Priestfield Road; Sisson, Battiste, Baxter, Wigmore, Holt, Hall, Gilbey and Roe all made their debuts.

[15] Gillingham lost 3–0 away to Southampton on 4 September, a game in which the correspondent for the Daily Herald described them as "hopelessly outclassed",[16] but then again beat Reading, after which they were 7th out of 22 teams in the league table, only one point behind leaders Portsmouth.

[15] The Daily Telegraph's reporter stated that the size of the defeat was more down to Gillingham's poor defence than the quality of their opponents' attack.

[11][19] Wood scored his first Football League goal for Gillingham a week later against Exeter City but his team lost for the fourth consecutive game,[15] after which they had slipped to 19th in the table.

Seven days later the team won their first Third Division match for more than three months when a goal from Needham gave them a 1–0 win at home to Bristol Rovers.

[30] Gillingham played Norwich again on 19 February and lost 2–1, and ended the month with a 1–0 defeat at home to league leaders Crystal Palace.

[33][34][35] The run of three league games without defeat was the longest the team had managed since the previous September, but they then lost consecutive matches to Queens Park Rangers and Swansea Town.

Gillingham entered the 1920–21 FA Cup at the fourth qualifying round stage, where they were drawn to play near-neighbours Maidstone United of the Kent League.

[15][40] Maidstone's goalkeeper saved two penalty kicks by Battiste but a goal from Gilbey meant that Gillingham beat their non-League opponents 1–0.

[15] It was each player's only appearance for Gillingham at Football League level, and in the case of Milton and Ollerenshaw the only game each played during their entire time with the club.

[15] FW = Forward, HB = Half-back, GK = Goalkeeper, FB = Full-back As a result of finishing bottom of the Third Division, Gillingham were required to apply for re-election to the Football League, but retained their place.