The 21-year-old arrived in West London from Rangers in the summer of 2017 and quickly rose through the youth ranks to make his debut for the club in August 2019.
Gilmour had previously scored eight goals and provided 10 assists in 40 outings for the Blues' Under-23 squad before becoming a fully-fledged member of Thomas Tuchel's first team.
However, the Scotland international would struggle to earn regular minutes in the Blues midfield amid competition from Jorginho, Saul Niguez, N'Golo Kante, Mateo Kovacic and Ruben Loftus-Cheek.
Gilmour was sent out on loan to Norwich City for the 2021-22 season and made a total of 28 appearances for the Canaries in all competitions, providing two assists in that time.
The midfielder turned out 24 times for Dean Smith's side in the Premier League - starting 21 of them - but he could not prevent the Carrow Road outfit from sliding back down to the Championship.
There was speculation surrounding an early recall for Gilmour after he went six top-flight games without playing a single minute for Norwich, and he also returned to Chelsea in January to undergo treatment for an ankle injury.
However, the ex-Rangers youngster returned to Carrow Road for the second half of the season and started four of their last five Premier League games before missing the final-day 5-0 loss against Tottenham Hotspur.
Gilmour's previous contract at Stamford Bridge was due to expire in the summer of 2023, with the youngster penning a four-year deal just after his debut in 2019, but he is now tied down to the club for another 12 months.
Since making his debut for the first team, Gilmour has pulled on the blue shirt 22 times at senior level, but he is still waiting for his first goal or assist for Tuchel's side.
The Scotland international has made five Premier League starts for the capital club in that time and was also involved on two occasions in Chelsea's 2020-21 Champions League-winning campaign.
Gilmour has also earned 14 caps for Scotland - starting seven of their World Cup 2022 Qualifying games before their playoff semi-final heartache at the hands of Ukraine.
Meanwhile, Chelsea were handed a potential injury concern with another midfielder over the weekend as Kante was forced to withdraw from the France squad due to a knee problem.