gas guns require you to rack the first round
AEGs and HPA conversions, for the vast majority, will load a BB as it brings the piston back.
So a GBBR will load AFTER it has fired a round, like real steel
An AEG or HPA conversion most often tends to load BEFORE it fires a round
Getting more complicated; precocking an AEG, or setting (for example) a polarstar HPA conversion unit to "closed bolt firing", will cause it to fire a round before loading.
All guns need to be discharged to prove they're empty. I know it's counter-intuitive to RS, but most guns don't have open access to the chamber, and gas guns that do provide access to the chamber don't have any easy method of removing the BB from the chamber. There's no extractor, so shooting is the easiest way.