The practise of leasing gold for short selling on the financial markets is not permissible. Leasing gold to someone who will sell it, is equivalent to lending. The lease payment is like an interest payment. This is the opinion of the scholars among the Hanafis, Malikis, and Hanbalis.
(Please note that, leasing gold for personal usage such as leasing gold jewelries may be subject to a different ruling)
And Allah knows best!
(Mufti) Billal Omarjee