Spanish outlet Cadena COPE are reporting that the deal is very close to being completed, with Gameiro expected to train with his new teammates in Valencia on Tuesday.
The fee for Gameiro is expected to be anywhere between €15m and €20m, with a three-year contract already agreed to.
The 31-year-old striker never managed to convince Diego Simeone that he was capable of leading the line in Madrid, scoring 27 goals in 82 appearances since joining in the summer of 2016.
Valencia are keen on Gameiro as he has extensive experience in Spanish football. He joined Sevilla from French side Paris Saint-Germain in the summer of 2013 and has been part of the league ever since. He represented Sevilla on 145 occasions and managed to score and impressive 67 goals.
However, he has always been a backup option in Madrid, with the likes of Antoine Griezmann and Diego Costa ahead of Gameiro in the pecking order. The Frenchman started only 11 games in La Liga last season, but completed 90 minutes on three occasions.
The likes of Zaza, Santi Mina and Rodrigo combined for an impressive 41 goals last season, with Zaza claiming 13 of those strikes. With Zaza set for a return to his native Milan, Gameiro will be required to fill the void.
He netted 11 goals in all competitions last season, but could see his tally increase next year with more playing time in Valencia.