How do I sel my BTC there at that price?
Be an accredited investor for which you'll need to prove you have a huge amount of play money then be patient for 12 months so you can sell.
You can't 'sell your BTC there'.
You have to buy or sell shares of GBTC which you own.
(IF you are an accredited investor and own coins within the trust, then yes, you have other options available to you.)
The shares I sold at $133.7 were shares I bought within my self-directed Ameritrade Roth IRA.
You do not have to be an accredited investor to buy or sell shares of this stock, whose value is backed by the number of bitcoins it holds.