Led by former Real coach Manuel Pellegrini, the Andalusian club are looking to overturn a 3-2 deficit from last week's first leg at the Bernabeu and prevent a quarter-final meeting between Real and 2011 runners-up Barcelona.
Barca, who lost 1-0 to their great rivals in last year's final thanks to Cristiano Ronaldo's dramatic extra-time winner, take a healthy 4-0 advantage to Osasuna for their second leg on Thursday.
"The cup is tough but you've got to believe," Gamez said after Saturday's 0-0 La Liga draw at home to Real's city neighbours Atletico.
"We saw the fans queuing and we know that the supporters are going to be behind us," he added. "If we managed that 0-2 lead at the Bernabeu, we can do it on Tuesday."
The side still alive after Wednesday's second leg will play Levante or second-division Alcorcon in the quarter-finals, with Alcorcon, who eliminated Real Zaragoza in the last round, leading 2-1 ahead of their game on Wednesday.
On the other side of the draw, second-division Cordoba have a 2-1 advantage over top-flight side Espanyol for Wednesday's game in Barcelona and third-tier Mirandes have a 2-0 lead when they play at Racing Santander on Tuesday.
