We present an advanced system for calibrating the detector gain responsivity with a chopped thermal source for POLARBEAR-2a, which is the first receiver system of a cosmic microwave background (CMB) polarimetry experiment: the Simons Array. Intensity-to-polarization leakage due to calibration errors between detectors can be a significant source of systematic error for a polarization-sensitive experiment. To suppress this systematic uncertainty, POLARBEAR-2a calibrates the detector gain responsivities by observing a chopped thermal source before and after each period of science observations. The system includes a high-temperature ceramic heater that emits blackbody radiation covering a wide frequency range and an optical chopper to modulate the radiation signal. We discuss the experimental requirements of gain calibration and system design to calibrate POLARBEAR-2a. We evaluate the performance of our system during the early commissioning of the receiver system. This calibration system is promising for the future generation of CMB ground-based polarization observations.