Owing to their high light absorption coefficient, excellent electronic mobility, and enhanced excitonic effect, two-dimensional (2D) GaN materials hold great potential for applications in optoelectronic and electronic devices. As the metal-semiconductor junction (MSJ) is a fundamental component of semiconductor-based devices, identifying a suitable metal for contacting semiconductors is essential. In this work, detailed first-principles calculations were performed to investigate the contact behavior between the GaN monolayer (ML) and a series of 2D metals MX2 (M = Nb, Ta, V, Mo, or W; X = S or Se). Despite the van der Waals (vdW) interface, the Schottky barrier heights (SBHs) of the MX2/GaN MSJs were found to deviate significantly from the Schottky-Mott limit. Stronger and weaker Fermi level pinning effects were identified in the higher and lower work function (WM) regions of the metals, respectively. This was attributed to the asymmetric charge redistribution-induced enhanced interface dipole (ΔP) in MSJs, leading to an increased step potential (ΔV) as response to the increased WM of MX2. p-Type quasi-ohmic contact could be realized in Ga-top stacking H-TaS2/GaN, H-NbS2/GaN, and H-VS2/GaN, indicating the potential application of 2D H-TaS2, H-NbS2, and H-VS2 as electrode materials. Applying biaxial tensile strain was identified as a feasible strategy for modulating the contact behavior in MX2/GaN, as it could effectively tune the SBH, change the contact type, or induce a transition from Schottky to quasi-ohmic contact. We demonstrated that strain effects on the contact properties of MX2/GaN MSJs were both MX2 and stacking configuration dependent, which were determined by the synergistic effect of the strain-modulated ionization energy and electron affinity of GaN ML, the WM of MX2, and the ΔV- and ΔP-quantified interface coupling in MSJs. Our work not only offers insights into the fundamental contact properties of 2D metal/GaN vdW interfaces but also provides strategies for electrode material selection and strain engineering to achieve ohmic contact and tunable SBHs in 2D GaN. This helps to provide theoretical guidance for the development of high-performance 2D GaN-based optoelectronic and electronic devices.