If this is what you want to carry.... Dress around the gun. Loose untucked shirts with bold prints or a jacket depending on the local weather can hide anything.
Next a high quality gun belt. Beltman or the like is the minimum for this size and weight firearm. Look at the total load out weight for the G21 with a loaded magazine in gun and one or two spairs, knife, phone or whatever.
Holsters: Another vote for Comp-Tac, but I would suggest the CTAC or MTAC IWB. While the Glock is big and thick, IWB with proper dress for the gun can hide just about anything. I have a CTAC for a different firearm it is one of the best. Many like the MTAC. I do not own a Raven Phantom, but this is also a well reviewed holster system.
''Concealed carry isn't supposed to be comfortable, it's supposed to be comforting.'' - Clint Smith