Western gorillas are endemic to western central Africa where they live; Gorilla g. gorilla is distributed over six or seven countries from south of Sanaga R. south through S. Cameroon, SW Central African Republic, mainland Equatorial Guinea (Rio Muni/Mbini), and Gabon, into extreme N. Angola (Cabinda enclave) to Congo R., and east into W. Congo and across the Sangha R. to about the Oubangui R. Western boundary is the Atlantic Ocean.
The north and the south border is roughly defined by the limits of the closed forest. Once resident in DR Congo, but probably now transitory in DR Congo. The extent of occurrence is roughly 700,000 km².
Gorilla g. diehli is the most northern and western subspecies of gorilla. Limited to the upper cross region on the Nigeria– Cameroon border about 260 km north of the range of G. g. gorilla, and about 250 km north of the recently discovered gorilla population in the Ebo Forest, Cameroon.
The extent of occurrence during the 1800s was 22,000 km² but the geographic range was greatly reduced to roughly 4000 km². There are at least 11 localities at which G. g. diehli occurs and these are believed to represent three subpopulations living in areas of 400 km² between Nigeria and Cameroon.
Recent molecular research indicates that at least occasional emigration of individuals occurs among the scattered subpopulations.
Gorilla g. gorilla is present in primary and secondary lowland forests, swamp forests, and submontane forests from sea level to at least 624 m. Especially common where ground vegetation is dominated by monocotiledoni plants such as Haumania liebrechtsiana, Megaphrynium macrostachyum, Raphia spp., and Sarcophrynium spp.
Gorilla g. diehli occurs from 200 to 2000 m and is now largely restricted to rough terrain in the submontane forests, although sometimes in lowland forests between hills, In the Cross River catchment area mean annual rainfall ranges from about 1800 to 4500 mm, and the mean monthly minimum temperature ranges from ca. 14 to 25 °C and mean monthly maximum temperature ranges from 24 to 36 °C,