1 Introduction  
1.1 Observation methods for the Galactic ionized gas  
1.2 Previous efforts in RRL observations  
1.3 Peering into the interstellar medium by FAST  

2 Processing the RRL data of the GPPS survey  
2.1 Pre-processing of the raw data  
2.2 Mitigation of the radio frequency interference  
2.3 Baseline fitting and RRL stacking  
2.4 Calibration and systematic uncertainties  

3 Results  
3.1 RRL emission from known H II regions  
3.2 New H II regions and distant star-forming regions  
3.3 Diffuse ionized gas  

4 Conclusions and discussions