The IMF have legal title to their gold holdings and the gold is...

The IMF have legal title to their gold holdings and the gold is initially paid by member countries into the IMF
as one-quarter of their capital subscriptions.

The IMF can get rid of gold in two ways -
i) IMF gold sales to its members for the
purpose of currency replenishment under the scarce currency provisions of Art. VII, Sec. 2 (ii) of the original Articles of Agreement.
ii) Under Article V, Sec. 12 of the IMF's Articles of Agreement, approval of gold sales by the IMF requires an 85%
voting majority.

Typically the gold sales are public auctions, and occur over a specified period of time so the markets are not flooded. The bidders are always gold market dealers/makers and the IMF dictate a lot of the rules, but it has historically been a bid price.

In the 70s, and over a 4-year period, the IMF sold a total of 23,517.5 thousand ounces (731.6 tons) of gold on competitive bids and 1,480.3 thousand ounces (46.0 tons) on
noncompetitive bids.

Prices tracked market prices, "sometimes somewhat above, sometimes somewhat below, the average fixing price of gold in London on the day of the auction."8 In the environment of worldwide inflation that characterized the late 1970s, gold prices were rising. Thus, the highest average price obtained in any IMF auction was $712.12 per ounce, occurring in February 1980; the official price of SDR 35 was equivalent to $46.07 on the date of the auction. The lowest average price, $109.40 per ounce, occurred in
September 1976, when the official price was equivalent to $40.42. Any return from the market sale of the IMF's gold in excess of the official price for its gold holdings of SDR
35 per ounce comprised profit.