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.
Isn't it kind of crazy to short an asset with such a low liquidity? I mean mostly actually offering the option to short such an asset. How do you enforce margin-calls, etc. if there aren't enough shares to be bought/sold at reasonable prices, to begin with???
I got burned pretty bad this way with GPRO, the price just kept going higher because of shorts being margin called while shares were still locked up.