The infiltration and growth (IG) process is an effective way for the preparation of bulk REBa2Cu3O7−x (RE-123) with finely dispersed RE2BaCuO5 (RE-211) particles compared with the conventional melt growth method. In this paper, single-domain Gd–Ba–Cu–O (GdBCO) bulk superconductors of 20 mm in diameter, with the same initial composition and cooling temperature window, but with different cooling times, have been fabricated in air by the IG technique. The morphology and levitation force of GdBCO samples are closely related with the cooling time in the same cooling temperature window of 1035°–1015°C. The results indicate that the growth region of single-domain GdBCO crystal increases with the increasing cooling time, and grows gradually up to the full sample size when the cooling time is >70 h. It is also found that the random nucleation of GdBCO grains can be effectively reduced with prolonging the cooling time. Furthermore, the levitation force of the samples is increasing with the increase of cooling time and nearly saturated when the cooling time reaches above 70 h. The results are very important and helpful for us to reduce the time and energy cost during the fabrication of GdBCO bulk superconductors.