7 GCA § 15602: Defense Founded upon Written Instrument.

When the defense to an action is founded upon a written instrument, and a copy thereof is contained in the answer, or is annexed thereto, the genuineness and due execution of such instrument are deemed admitted, unless the plaintiff files with the clerk, within ten (10) days after receiving a copy of the answer, an affidavit denying the same, and serves a copy thereof on the defendant. (If the defendant relies upon a written instrument, in whole or in part, that fact shall be pleaded.)