Many tripods allow removal of the entire head from the column, so a new head with QR can be fitted.
Other tripods have heads with removable baseplates at the top of the head, so threaded baseplate can be removed and fitted with a QR base
Sometimes removal of the existing head/baseplate entails loosening a setscrew on the underside.
Look for either removeable baseplate or removable head in your Gitzo. You might let us know which specific Gitzo head is on the tripod, or which Gitzo tripod. Likely someone has experience with that specific unit.