As CDA indicated there are different "combustibles", liquids, solids, dusts, fibers. Also, which code?
IBC uses the ASTM 136 test, but also allows "composite materials" to be accepted as non-combustible if the base material is non-combustible as long as the facing is less than .125" and a flame spread of 50 or less. This allows gypbd to be considered non-combustible by the IBC, otherwise a ASTM 136-only test would render gypbd as combustible.
IMC does not have the composite exception, therefore gypbd is considered combustible for things addressed in the IMC.
NFPA 101 has a category for "Limited Combustible" which is similar to the composite exception in IBC.
As gb indicated, this is a very tough test, and many common building materials not capable of igniting or burning do not meet it or are not suited to be tested to that standard (aluminum).
For liquids, NFPA, IBC, and IFC liquids having flash point at or over 100 deg F are combustible. Under 100 deg F is considered Flammable.