  Gaseous X-ray polarimetry refers to a class of detectors used for measuring
the polarization of soft X-rays. The systematic effects of such detectors
introduce residual modulation, leading to systematic biases in the polarization
detection results of the source. This paper discusses the systematic effects
and their calibration and correction using the Gas Microchannel Plate-Pixel
Detector (GMPD) prototype for POLAR-2/Low-Energy X-ray Polarization Detector
(LPD). Additionally, we propose an algorithm that combines parameterization
with Monte Carlo simulation and Bayesian iteration to eliminate residual
modulation. The residual modulation after data correction at different energy
points has been reduced to below 1%, and a good linear relationship is observed
between the polarization degree and modulation degree. The improvement in
modulation degree after correction ranges from 2% to 15%, and the results
exceed those of the Imaging X-Ray Polarimetry Explorer (IXPE) above 5 keV.
