The England international, 24, was pictured in the La Liga club’s Metropolitano Stadium on Saturday and spent five days in a hotel in Madrid but has been unable to complete his £33m transfer.
His move was halted because Chelsea’s £34.5m deal to sign Spanish striker Samu Omorodion collapsed over personal terms on Sunday.
The Spanish club need to make one major sale to fund their move for Gallagher after signing Manchester City striker Julian Alvarez for £81.5m on Monday.
Gallagher’s move is not considered off but remains under threat of collapse.
An alternative deal for Chelsea to sign Portugal forward Joao Felix from Atletico could salvage Gallagher’s move but a fee must first be agreed.
Felix is ready to return to Chelsea after spending six months on loan at the club in 2023.
However, if a fee cannot be agreed for Felix then Gallagher’s transfer will likely collapse.
Chelsea want to make the sale to avoid losing Gallagher on a free transfer, with just 11 months left on his contract.
Gallagher, who captained Chelsea for much of last season, made five appearances at Euro 2024.
On Saturday, Atletico had posted a picture of Gallagher at their stadium saying he was there to “finalise his transfer”.
