You have the capability to send a received fax to a case directly from the Fax tab in Gardiant.
Go through the Fax tab > Received sub-tab, find the fax that you want to use (you can read the received fax by clicking on the PDF link under the File Name column), click on the ellipsis at the beginning of the row, and select the Sent to Case option in the menu:
This will open the Send to Case window where you can search for the case by the claimant name, the claim number, the lead, or narrow results by the case status:
If you change your mind and no longer want to send the received fax to the case, you can click on the X button in the top, right-hand corner of the Search for a case window to close it.