For over a year, speculation has persisted that co-owners Todd Boehly and Clearlake Capital have been willing to cash in on the academy graduate to bolster the club's position with the Premier League's Profit and Sustainability regulations.
Gallagher survived at his boyhood club for the first two transfer windows, with Mauricio Pochettino heavily relying on the England international to lead his Blues side in the absence of injured duo Reece James and Ben Chilwell.
However, despite producing the best season of his career in 2023-24, it quickly became apparent that Gallagher would not be given the same level of responsibility under Enzo Maresca.
Although a transfer to Atletico has been delayed for over a week, there has been an inevitability that the 24-year-old would finalise his switch to Spain.
That time came on Wednesday morning, Chelsea confirming that Gallagher had moved to Civitas Metropolitano with a video that paid tribute to the service that he had given them.
How did transfer pan out?
As per transfer expert Fabrizio Romano, Atletico have paid Chelsea a €42m (£35.85m) fee, the expectation being that Joao Felix will move in the opposite direction in a separate deal.
Both clubs have been wanting to improve their respective financial positions, Gallagher's switch to Atletico the least complex to complete with Chelsea making pure profit on the deal.
From the Spanish side's perspective, however, they need to generate a fee that ensures that they do not lose out. The breakdown of Samu Omorodion's switch to Chelsea meant that all parties had to react.
That ultimately led to Felix - a former Chelsea loanee - entering the conversation at a time when Atletico were already eager to cash in on their club-record signing.
Chelsea have allegedly agreed to pay £44.5m for the Portugal international, negotiations having been drawn out for around nine days courtesy of Atletico needing to recoup a minimum fee to ensure that they do not make a loss from an amortisation viewpoint. That transfer will be confirmed later on Wednesday.
Gallagher pushed out by Chelsea
Gallagher may have been prepared to spend his entire professional career at Chelsea, a club that he joined as an eight-year-old back in 2008.
Instead, he spent just two seasons in the first team at the West Londoners, contributing 10 goals and 10 assists from his 95 appearances in all competitions.
As a player with a further 92 outings in the academy setup, Gallagher appeared to be the ideal candidate to captain Chelsea for years to come.
However, that responsibility was taken away from him for financial and playing-style reasons. It remains to be seen whether Chelsea will come to rue that decision, regardless of the immediate breathing space it creates in their accounts.