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.