World Cup qualifiers: Dani Carvajal becomes first Real Madrid player to reach Luis Enrique's Spanish squad since March
The other additions are AC Milan attacking midfielder Brahim Diaz and Barcelona teenager Ansu Fati.
Gaya was substituted in the 75th minute of the 4-0 win over Georgia on Sunday because of a muscle injury.
The only players from non-Spanish clubs in the Olympic squad are Olmo of Leipzig and Rafa Mir of Wolverhampton Wanderers.
Spain's preparations have been heavily disrupted after captain Sergio Busquets and defender Diego Llorente both tested positive this week.
Captain Sergio Ramos, the 35-year-old Real Madrid captain has struggled with injury this season and has featured just five times since the start of the year.
First choice keeper de Gea was badly at fault for Portugal's second goal of the game when he let a long-range shot from Portugal captain Cristiano Ronaldo squirm from his grasp.
Untested Spanish coach Fernando Hierro hopes to repeat Zinedine Zidane's coaching success in FIFA World Cup 2018
Hierro is taking on a major coaching job without any significant previous experience, just as Zidane did at Real Madrid.
Coach Lopetegui successfully blended emerging talent with the experience provided by veterans to qualify for an 11th straight World Cup.
Spain remained undefeated through 20 matches under coach Julen Lopetegui.
A statement released by the national team on Monday indicated the 26-year-old will remain with the squad.