The Z machine at Sandia National Laboratories was recently refurbished to improve capacity, precision, and capability of nearly all of its pulsed power components in each of its thirty-six modules. The goal of this upgrade was to increase the total machine current by 30%, from ~20 MA to 26 MA in 120 ns into a wire-array z-pinch load. Initial design calculations indicated the operating point for an individual gas switch would be 5 MV and 600 kA to meet these design goals. This manuscript discusses design improvements that were required to achieve reliable performance at this level, the highest requirement for an LTGS in history. Significant factors include the electric potential grading along the switch length, the materials used in the switch, and the insulator housing design. It also discusses design criteria for robust SF6 gas switch design. Several design iterations are discussed that address performance improvements for each failure mode.