Reducing the Schottky barrier at the metal-semiconductor interface and achieving Ohmic contacts are very important for developing high-performance Schottky field-effect devices. Based on the fact that GaN and 1T-VSe₂ monolayers have been successfully prepared experimentally, we theoretically construct a GaN/1T-VSe₂ heterojunction model and investigate its stability, Schottky barrier property and its modulation effects by using first-principle method. The calculated formation energy and the molecular dynamics simulations show that the constructed heterojunction is very stable, meaning that it can be realized experimentally. The intrinsic heterojunction holds a p-type Schottky contact and always keeps the same p-type Schottky contact when tensile or compressive strain is applied. But when the external electric field is applied, the situation is different. For example, a higher forward electric field can cause the heterojunction to change from a Schottky contact into an Ohmic contact, and a higher reverse electric field can lead to a variation from a p-type Schottky contact to an n-type Schottky contact. In particular, by implementing chemical doping, the transition from Schottky contact to Ohmic contact can be achieved more easily for the heterojunction. For example, the introduction of B atom enables the GaN/1T-VSe₂ heterojunction to realize a typical Ohmic contact, while for C and F atom doping, the GaN/1T-VSe₂ heterojunction can achieve a quasi-Ohmic contact. These studies provide a theoretical reference for the practical application of the suggested heterojunction, and are of very important in designing novel high-performance nano-scale electronic devices.