The "Turbo Titan" was one of several GM test bed units and this was operational from around 1968. Power was a modest 280-hp and GM claimed that this was a highly developed version of earlier prototype models. The GT-309 GT drove through a six speed automatic transmission.
That was the Chevrolet Turbo Titan III, which came out in late 1964.
Previously, Chevy had the Turbo Titan I of 1956, and the Turbo Titan II of 1959. These were built in stock Chevy heavy duty conventionals, and were equipped with the GT-304 and GT-305, respectively.
1965 saw the GM Bison concept, which was powered by the GT-309, much like the TT III.
In 1969, a GT 309 was installed in a GMC Astro 95, and a similar experiment was repeated in 1979, but with a GT-404.
GM was also active in putting gas turbines in experimental buses. 1954's TurboCruiser I sported a GT-300, and 1964's TurboCruiser II was equipped with a GT 309. This coach was improved and re-built as the TurboCruiser III in 1969, albeit I think it still used the GT-309 at that time. I've not been able to confirm or deny that at this time.
The GT 309 was also used in the experimental GMC R(apid) T(ransit) (e)X(perimental) of 1968, coupled through a prototype Allison Toric transmission. This bus was the predecessor to the 1973 Transbus submission, which too was turbine powered - albeit with a GT-404.
In '71, the GT-404 was installed in a few MCI MC-7 coaches owned by Greyhound, and named "Super Turbocruisers". In '79, Baltimore operated one RTS-II 03 coach equipped with a GT-404, while the remainder of their order was powered by conventional Detroit Diesel power.