In this article, the gate current in AlGaN/GaN high-electron mobility transistors is modeled in a surface potential-based compact model. The thermionic emission, the Poole-Frenkel emission, and the Fowler-Nordheim tunneling are the dominant mechanisms for the gate current in the forward-and reverse-bias regions. These conduction mechanisms are modeled within the framework of the ASM-GaN compact model, which is a physics-based industry-standard model for GaN HEMTs, hence yielding a consistent model for the drain and gate currents. The proposed model captures the gate voltage, drain voltage, temperature, and gate-length dependencies of the gate current. The results of dc gate-leakage measurements of two GaN HEMT, differing only in terms of gate length, over a wide range of temperature, showing these current-conduction mechanisms, are presented, and the proposed model is validated accordingly. The developed gate current model, implemented in Verilog-A, is in excellent agreement with the experimental data.