What makes gorillas and other apes so incredibly strong is not really their muscle mass but their muscle composition: Their fast-twitch-slow-twitch-musclefiber-ratio is much more skewed towards fast-twitch-fibers, which grants them greatly increased explosive strength at the expense of endurance in comparison to humans.
Therefore, if you want strength, the muscle fiber composition is the first and easiest variable you can play with.
When it comes to muscle mass then, the easiest thing would be to take a look at our own strongest specimen: Strongmen.
Looking at them provides us with several obvious challenges with regard to the humanoid frame that you have to accomodate for in one way or another:
Muscles burn an enormous amount of energy and require specific nutrition like protein/aminoacids for proper maintainance, especially when they are put to use regularly:
Strongmen consume multitudes of the 2000 kcals that a regular human requires, up to or even beyond 10000 kcals per day.
Given the fact that you want your mutants to be even bigger and heavier than our biggest specimen, they will require even more nutrition.
With nutrition comes digestion: Our strongmen do already eat basically the whole day, which means your mutants will definetely require a significantly better digestion system than we have.
Joint and back problems: While muscles support our bones in carrying our body weight, they are only advantagous in that regard until a certain point: Weighing 150 kilos and more is not healthy, even when all the weight comes from muscles. The strain that weight and height can put on joints, knees and spine is enormous and will have to be accomodated for in some fashion.
The human frame simply is not built for being exceedingly heavy and tall.
The possible solutions to those challenges depend on how human-like you want your mutants remain and how much you want to handwave (unnaturally resistent bones and joints, superefficient metabolism etc...).
