The remote epitaxy of GaN via graphene has attracted much attention due to the potential of easy mechanical exfoliation, and the exfoliated layers can be transferred onto foreign substrates according to the application needs, which is beneficial to improve the performance of GaN-based devices. In this work, a GaN epi-layer was grown by metal-organic chemical vapor deposition on the monolayer-graphene-coated AlN/sapphire or GaN substrates. The influence of growth temperature, carrier gas, and substrate on the exfoliation of the GaN epi-layer was studied. When the growth temperature is no more than 800 degrees C and N2 is used as the carrier gas, the monolayer graphene can be retained on the AlN/sapphire substrate during the growth process. Thus, the GaN epi-layer can be exfoliated successfully. However, the monolayer graphene will be destroyed under a growth temperature of 850 degrees C, and lead to the failure of exfoliation. Besides, the monolayer graphene can also be damaged when the H2 carrier gas or GaN substrate is employed with a growth temperature of 800 degrees C. This causes the GaN epi-layer to be exfoliated not as well. The experimental results illustrate that suitable growth conditions and substrate are important for realizing the exfoliation of a GaN epi-layer.