The world number 34, who reached the quarter-finals in the Catalan city last year, looked out of form and departed the tournament after just one hour and 47 minutes.
Giraldo will next face wildcard Albert Ramos after the Spaniard came from a set down to beat compatriot Ruben Ramirez Hidalgo 4-6 7-6(5) 7-5 in three hours and 11 minutes.
Eleventh seed Albert Montanes enjoyed a more successful day as he beat Teymuraz Gabashvili 6-2 6-2 to book his place in the second round.
Montanes will face qualifier Benoit Paire in the next round, after the Frenchman continued his solid run of form to beat Pablo Carreno 6-7(7) 6-3 6-2.
Romania's Victor Hanescu broke Andrey Golubev's serve four times as he beat the Kazakh 6-3 6-2 to progress to a clash with Kevin Anderson after the South African battled past Pablo Cuevas 7-5 3-6 6-4.
The top eight seeds receive a bye through to the second round top seed Rafael Nadal due to get his tournament underway on Wednesday.