Goal understands that Los Che have struck a deal with Guedes, who enjoyed a successful loan spell at the club during the 2017-18 season.
Valencia and PSG are now in negotiations over the details of a transfer that is expected to reach a total of €50 million (£45m/$57m).
One possibility is an immediate transfer which includes 15-20 per cent of any future deal, while Guedes could also spend another year on loan at the Liga club before penning an identical deal.
Guedes played 33 times for Valencia during last season's Liga campaign, scoring on five occasions.
Gameiro, too, is set to swap Madrid for Valencia after Los Che confirmed they had reached a preliminary agreement for the French striker.
While the club declined to reveal the fee agreed with Atletico, reports suggest Gameiro will move for an initial €16m (£14m/$18m) with a further €2m payable later in performance-related bonuses.
Comunicado oficial https://t.co/TpwAMXNE11— Valencia CF 🦇💯 (@valenciacf) 12 de agosto de 2018
The forward moved to the Spanish capital in 2016 from PSG, signing a four-year deal with the side coached by Diego Simeone.
But he failed to keep up the momentum of an encouraging debut season, and in 2017-18 was a bit-part player for the Colchoneros behind the likes of Antoine Griezmann, Angel Correa and Fernando Torres.
Gameiro made 25 Liga appearances last term, scoring just seven goals.