The $BTC getting locked up on $GBTC are not redeemable so they're effectively locked/off the market.
More coins going off the market than are being mined.
1. People send money to Grayscale, Grayscale buys them. According to the Messari research this is ~30% of coins mined after the halving
2. OGs/Funds send their coins to GBTC to take advantage of the premium. This has 6 month lockup
2.b. OGs/Funds dump the shares after 6 months when the lockup period expires if there's a premium and rebuy BTC.
2.b.2. the same amount of BTC and keep premium in cash.
Also the people buying $GBTC are not likely to buy actual Bitcoin with that money.