A G20 (also G29 & G40) can shoot 40sw with no changes. The 40sw cartridge will headspace off the extractor that can subject the extractor to undue stress. In my opinion, that is not a reliable way to do that.
The G20,G29,G40 can use conversion barrels made for those models but chambered for 40sw so the cartridge will properly headspace off the case mouth. I have several KKM barrels for glocks and no problems with them.
The glock 10mm mags will feed 40sw and stock recoil springs will work fine. All i do is swap barrels.
Or just buy normal 10mm rds stay away from Underwood, Buffalo Bore, Definitely Double Tap, and Sig is producing good 10mm rds now. About every other manufacturer producing 10mm is pretty much reduced to 40 S&W already.
Federal 10mm Auto 180 gr Hydra-Shok JHP 1030 fps
Federal 10mm Auto 180 gr FMJ American Eagle 1050 fps
CCI 10mm Auto 200 gr FMJ Blazer 1050 fps
Armscor 10mm 180 Gr Full Metal Jacket 1008 fps
Now full power 10mm
Buffalo Bore 10mm 180 gr JHP 1350 fps
Underwood 10mm Auto 180gr. Bonded Jacketed Hollow Point 1300 fps
People are gonna do what they want As a Glock Armorer Do not shoot 40 in a Glock model 20 UNLESS you change to a designated 40 barrel.
As mentioned it it will cause undue stress on parts. It doesn't that people know people that "do it" Glock recommends against it. And most people do not know more than the engineers that makes the guns. And just because you can doesn't mean you should!