Lieutenant Francis "Frank" Gabler was a 23rd century Starfleet officer serving aboard the USS Enterprise under Captain James T. Kirk. He was one of Montgomery Scott's best engineers, who also doubled as a security officer and a helm officer. His apparent predecessor as Scott's Assistant Chief Engineer, at least in TOS, was Lieutenant DeSalle(Michael Barrier).
In 2269, when Scott and Kirk carried a force field box, containing an antimatter villi from an unidentified cosmic cloud, into the nacelle of the Enterprise, Gabler assisted Scott remotely, by opening and closing the doors to an area at the rear of the nacelle where the antimatter was placed. (TAS: "One of Our Planets Is Missing")