The midfielder begun his journey to stardom at local side Epsom Eagles before the Blues snapped him up aged eight.
Gallagher impressed in the club's academy and eventually earned a call-up to the England Under-17s.
He was part of the side that sealed World Cup glory for the Young Lions in 2017 alongside Phil Foden, Callum Hudson-Odoi, Morgan Gibbs-White and Jadon Sancho.
Gallagher was rising through the ranks and pushing to make his first senior appearance when his career was nearly taken away from him.
In the summer of 2018, while in pre-season training with Chelsea, the then 18-year-old felt his heart beating strangely fast.
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