After some quick research,
a Lion don't want no smoke from a Gorilla.
The largest gorilla you will most likely find in the wild is 525 lbs - 550 lbs and 6ft 3 - 6ft 6 with a 8ft - 9ft wing span
The largest lion would be around 525 lbs - 575 lbs with around a 7ft standing height with a 4ft limb length
An absolute beast gorilla:
Intelligent enough to manipulate its environment, use tools, learn sign language and so on
Run at 30mph and is agile
Lift 1500lb easily with one arm. lift over 3000lb above their head and throw it
Apparently jump 8 ft (never officially recorded)
Bite force over 1300 psi (more than a lion)
Has huge forearms so powerful that they can grip and hang from a tree with one arm for hours
Pull with a force over 5000lbs.
A beast lion:
Dumb as rocks except for advanced pack hunting tactics
Run at 50mph but only for a very short while and would be exhausted
Jump 9ft high
Bite force of over 600 psi
Pull 1000lbs
Slap a 2000lb Ox to Bolivia
Seeing as a gorilla has hands, and feet that are similar to hands, they can utilize its strength more. A gorilla has Jui Jitsu, wrestling and boxing. How can a Lion counter that? (Not really but you know what I mean) Plus Silverbacks are just older gorillas so they have that old man strength and fighting experience.
A Lion has few fighting tactics and can only exert itself for a very small amount of time, which is why they ambush and try to immobilize their prey quickly. A lion would stick its claws in and try to bite the neck to either suffocate, break the spine or just destroy it. Have you seen Gorilla traps
If the Gorilla somehow let a lion get to that position, a 550lb Gorilla , it would reach back before any bite, lift the lion up and literally tear him into pieces.