If your great grandfather listed his occupation as a hostler – what did he do?

The word is spelled "hostler" in American English, but "ostler" in British English. It traces to c.1386, meaning "one who tends to horses at an inn"—and also, occasionally, "innkeeper." It is derived from Anglo-French hostiler, itself from Medieval Latin hostilarius "the monk who entertains guests at a monastery," from hospitale "inn".

Modern Usage: According to the Dictionary of Occupational Titles, a hostler in motor transportation is a type of truck driver who directs trucks or tractors at vehicle parking or docking areas to move, position, or park trucks or trailers. In the United States railroad industry, a hostler is a type of railroad engineer who drives electric, diesel-electric, steam, or gas-turbine-electric locomotives to designated stations in a railroad roundhouse to be cleaned, serviced, or repaired.