"Gold Tranche" (Reserve Tranche) refers to

A reserve tranche, also known as the "Gold Tranche," is a portion of the required currency quota that each member country must provide to the International Monetary Fund (IMF).

This tranche can be utilized by the member country for its own purposes without a service fee or economic reform conditions.

The reserve tranche represents the member’s unconditional drawing right on the IMF, created by the foreign exchange portion of the quota subscription, plus increases (decreases) through the IMF’s sale (repurchase) of the member’s currency to meet the demand for the use of IMF resources by other members in need of balance of payments financing.

Essentially, it's an emergency account that IMF members can access at any time without agreeing to strict conditions or paying a service fee.

The reserve tranches that countries hold with the IMF are considered their facilities of first resort.

Hence, Gold Tranche is a credit system granted by IMF to its members.