A review of "Operation of 8500-Hp Gas Turbines in Locomotive Service" by Harold Rees, Chief Mechanical Officer, Union Pacific Railroad, October 1960, shows that the original specification for turbine fuel was for a residual fuel that would flow at a viscosity of 95 SUS at 210 degrees Fahrenheit. The specification matches the American Society for Testing and Materials (ATSM) standard known as Number 5 fuel oil, also known as Bunker B. (Read the Wikipedia article about the definition of SUS, used as a measure of viscosity)
So, Union Pacific used No. 5 heavy fuel in its gas turbine locomotives, not Bunker C. No. 5 heavy fuel (Bunker B) was a slightly higher grade of residual fuel than the Bunker C used in steam locomotives. As mentioned in Rees' report, burning residual fuel in a gas turbine was different than burning residual fuel in a steam locomotive, and the higher grade fuel was needed to control the ratio of non-combustible, destructive by-products in the fuel itself.
Bunker C is simply a generic name for one of several heavy residual fuels. Railroaders were not chemists. Many were veterans of the U. S. Navy, where the term "Bunker C" was very common. The black heavy fuel used for the Gas Turbines looked and acted like the black fuel used in ships and in steam locomotives. Right or wrong, they continued to use Bunker C as the name of the nasty black, residual fuel that made their daily lives so difficult. And railfans tend to use whatever names railroaders use.