Gas spark closing switch is one of the most important components for a fast linear transformer driver (FLTD). In this paper, we describe the investigation of insulation gas media for a multi-gap multi-channel gas spark closing switch. A six-gap gas switch with corona needles was tested in a typical FLTD brick with two capacitors of 40 nF and a load resistor of about 10 Ω. Corona discharge current, self-breakdown voltage distribution, and triggered breakdown performance were tested when the gas switch was filled with air, N2, CO2, SF6/N2, and C4F7N/N2. When C4F7N/N2 was applied, there was no abnormal breakdown with low voltage found in the whole test process; the trigger breakdown delay time and switch jitter were very stable, and no pre-fire was found during about 2000 triggered shots. Therefore, we think the C4F7N/N2 gas mixture with a very little amount of C4F7N can dramatically improve the switch performance. It is valuable and easy to realize the application of C4F7N/N2 mixture in large scale pulsed power facilities.