Gorillas have 2.22x as much muscle as humans. 73kg vs 33kg.
Their arms are 37% of their muscle mass, humans are 20%. Making their arms 1.85x bigger by proportion.

How much muscle, in kg, are in humans and gorilla arms?

Sources:
https://onlinelibrary.wiley.com/doi/full/10.1002/ar.21449
https://www.physiology.org/doi/full/10.1152/jappl.2000.89.1.81
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4475937/#s11