Male Gorilla can Carry a Large Man.
Stories of great apes being 10 times as strong as people, and gorillas being able to bench-press a Land Rover, seem to come from sloppy pop-science writers citing the 1926 paper of Bauman. That paper was a hundred years ago and should set off alarm bells for the scrutiuous reader.
This paper from 2017 is about examining the muscles of chimps from an interior point of view. i.e what about their muscles and anatomy makes them so strong. Good for you it also contains a literature review of existing direct tests of great ape strength, in Section II of the supplemental material. Go to this page and scroll down to Supplementary Material. Table S6 comes from that review.
In particular the review mentions the most recent study on Chimps and Macaques that measured normalised strength for a full-body pulling task. The animals can pull a rope and can anchor using their arms and legs. See movie
They got the following results:
In this particular task, Chimps raised in captivity are about twice as strong as athletic humans, relative to their body weight.
This does not measure how good is a chimp at carrying a large inert mass over long distance, let alone a gorilla $-$ even if we distribute the weight over both leg and arm strength like in the study. However it looks like the best number we have.
We will assume an untrained chimp can carry a backpack twice as heavy relative to body weight, compared to a trained human. We will take the same number for your gorillas.
We will ignore any increase in strength by training the gorillas. In the other direction, we will ignore any drop caused by the cube-square law, since gorillas are bigger than chimps. However note the above plot goes against a naive application of the cube square law, since the chimps are proportionally stronger than macaques despite being bigger. This is because macaques are lanky and chimps are super-studs.
Homework: Is the average Lowland Gorilla(Gorilla Gorilla) more or less studly than the average Chimpanzee(Pan troglodytes)?
So how heavy is a gorilla? Wikipedia says "Wild male gorillas weigh 136 to 227 kg while adult females weigh 68–113 kg."
Now males of the species are usually stronger proportionately, but we suspect the upper range is for Silverbacks who are big for display and fighting other Silverbacks, rather than for practical reasons. So we will take the ape steed as 150kg.
A trained human soldier can carry 100lbs or 45 kilos of equipment for hours at a time. If the human is Michael Phelps:
whose weight is advertised at 88kg, and who wears the American flag as undies, then the pack is about half his body weight.
Since the gorilla is about twice as strong by weight, the gorilla can carry about one times its bodyweight. Or an entire second gorilla, suitably distributed across his body.
This already sounds unlikely. Remember the soldier's equipment is not all in the backpack. It is distributed across the body so the soldier can walk without falling over.
But fortunately the gorilla does not need this much to strength to carry a Phelps. If the gorilla weights 150kg and the Phelps weighs 88kg then he is only carrying 60% of his bodyweight. This sounds more believable.
If we replace the Phelps with a Tom Cruise or Geena Davis, then the ape can carry two 10kg baskets of fruit along with the rider. If we replace the Phelps with an average-size Natalie Portman then the ape can carry two 18kg baskets of fruit.
But can they Climb?
The above is about carrying a backpack, with weight correctly distributed over four limbs. It is not about climbing through the trees. When climbing the gorilla sometimes uses only one arm:
Photo by Jonathan Rossouw
I cannot imagine this kind of fancy manoeuvre is possible if the gorilla is carrying Michael Phelps on its back. In fact I cannot find pictures of ANY adult gorilla hanging like this. That throws another spanner at the idea of gorillas being good at climbing.
Maybe if the trees are big enough to run along the branches on all fours like in Avatar:
with the occasional jump to the next branch. Then your gorillas will work.