how can we short GBTC?
You can't.
I don't think one can go short yet. That constraint should get lifted one month from the date it started trading. There are probably a few hedge funders salivating at the opportunity to short those crazy-high bids.
Also, there is probably a lot of interest from accredited investors to go directly to the trust and buy these shares at par. Those shares in turn will be available for trading in the open markets a year from now.
As an insurance plan for whenever they allow shorts you can calculate:
(12 month interest fees for borrowing shares to short + price of buying new shares at current rates) as your cost.
Sort of like a 12 month arbitrage plan.