I will be doing it with current reported sales to Gaon and also to Gaon+Oricon
data compiled by K-Pop Artist Sales @koreansales_twt - Google Sheets from Gaon and Oricon's sites
HYBE Labels (Bighit, Pledis, BELIFT, KOZ)
Artists included for HYBE LABELS = BTS, TXT, Enhypen, Seventeen, NU'EST, fromis_9, Zico
Overall artists included = 7 acts
Gaon Sales = 15,718,597
Gaon + Oricon Sales = 18,000,087
Artists included for YG Ent = BP, Lisa, Rose, Treasure, Bigbang,AKMU,iKON,Mino, Jennie,Winner,Sechskies,Bobby
Overall acts included = 12 acts
Gaon Sales = 2,441,813
Gaon + Oricon = 2,453,468
Artist included for JYP = TWICE, SKZ, ITZY, Day6, 2PM
Overall acts included = 5 acts
Gaon Sales = 5,517,489
Gaon + Oricon Sales = 5,917,459
Artist included for SM Ent = Aespa, NCT units, SNSD, EXO, EXO-SC/CBX, EXO members, SNSD members, Shinee, Super Junior, Shinee members, SJ members, TVXQ, Red Velvet, RV members, SuperM etc.
Overall acts included = 36 acts
Gaon Sales = 16,761,812
Gaon + Oricon Sales = 17,086,530
Thus,
The best selling company in 2021 on Gaon was SM (36 acts) with 16.7M sales
2nd best selling was HYBE (7 acts) with 15.7M sales
The best selling company in 2021 on Gaon+Oricon was HYBE with 18M sales
2nd best selling was SM (36 acts) with 17.1M sales