Leopard predation on chimpanzees (over five years) is reported in the Tai forest and includes nine likely attacked individuals, but none directly observed and only one leopard scat with chimpanzee bones and hair [Boesch, 1991; Zuberbühler and Jenny, 2002]. Leopards in Tai were once reported to scavenge a chimpanzee carcass [Jenny and Zuberbhler, 2005]. Evidence of leopard predation on chimpanzees (6/196 leopard scats) has also been recorded in Lope, Gabon [Henschel et al., 2005].
Evidence of leopard predation on other great apes is rare, although Fay et al. [1995] described a probable leopard attack on lowland gorilla including a digit in leopard scat and recent systematic studies of leopard scats in sympatric areas have revealed evidence of predation on gorillas [9/196 scats: Henschel et al., 2005] and bonobos [1/5 scats: D’Amour et al., 2006].
Rates of leopard predation on apes therefore range up to 5% of scat samples.
http://ugallaprimateproject.com/files/StewartPruetz2013-AJP.pdf
Apart from humans, gorillas do not really have enemies.
The only predator to prey on gorillas is the leopard. Walter Baumgärtel found the remains of several gorillas after they had been killed by leopards in the Virunga Volcanoes. Other hints were found in Gabon and the Central African Republic (Tutin & Fernandez 1991; Fay et al. 1995)
http://www.angela-meder.de/publik/eep.pdf
As large, group-living primates, gorillas have few natural predators.
There are some records of leopards killing adult gorillas (e.g. Baumgartel, 1976) and young gorillas could potentially be taken by pythons or eagles, but infants are normally protected by the adults
http://www.unep.org/pdf/GorillaStand_screen.pdf
1995. w. R. Carroll, JCK Peterhans, et al.
Leopard attack on and consumption of gorillas in the Central African Republic. Journal of Human Evolution 29(1): 93-99
http://www.umich.edu/~infosrn/PDF_FILES/ENGLISH_.PDF/SEC._4/FAY.PDF