The Bexar County Medical Examiner's Office in Texas tells TMZ ... John died of cardiomyopathy and chronic obstructive pulmonary disease -- heart disease and COPD, in layman's terms -- complicated by fentanyl toxicity.

The M.E. also says the overdose was accidental.

As we reported ... John died back in November after he was found unresponsive in a San Antonio hotel room.

It was a tumultuous 2020 for the Getty family. Just 2 months before John died ... his mother, Ann, died from a heart attack. John's other brother, Andrew, died in 2015 from meth intoxication.

John Gilbert, of course, was one of several descendants of J. Paul Getty -- the oilman whose art collection was central to the famed Getty Museum in Los Angeles.