We demonstrate the key module of comparators in GaN ICs, based on resistor-transistor logic (RTL) on E-mode wafers in this work. The fundamental inverters in the comparator consist of a p-GaN gate HEMT and a 2DEG resistor as the load. The function of the RTL comparators is finally verified by a undervoltage lockout (UVLO) circuit. The compatibility of this circuit with the current p-GaN technology paves the way for integrating logic ICs together with the power devices.