You can carry the G21. All you have to do is get a holster with lots of forward cant so the bottom of the grip does not print. Look at the El Paso Saddlery Dual Duty holsters. They have extreme forward cant. Also consider the DeSantis Cozy Partner IWB holster.
I have the DeSantis, feels amazing. It's the hybrid. Half kydex and half leather. Forget it's even there. I have my SIG P226 with my beefy hogue grips (big but not beefy hands) on it and it conceals just fine in a t-shirt.
It fits my Glocks as well as my PPQ and a few others. I know..how versatile. It fits them safely.
Sáncte Míchael Archángele, defénde nos in proélio, cóntra nequítiam et insídias diáboli ésto præsídium. Ímperet ílli Déus, súpplices deprecámur: tuque, prínceps milítiæ cæléstis, Sátanam aliósque spíritus malígnos, qui ad perditiónem animárum pervagántur in múndo, divína virtúte, in inférnum detrúde. Ámen