How do the nutrients in dead matter become available to leaves?

When something, whether its animals or plants dies, they are broken down by microorganisms (decomposers). These microorganisms are also called saprobionts. These secrete enzymes onto dead tissue, this is called extracellular digestion. They absorb products of digestion. Respiration by microorganisms produces carbon dioxide. Carbon dioxide is then taken into the leaves.

How do gorillas raised in captivity know what's safe to eat in the wild?

By instinct. Similar to how we have evolved to enjoy the taste of the safe foods and dislike the poisonous ones, gorillas have done just the same. In the wild mountain gorillas eat leaves,shoots and stems so much so an adult male can chomp through a massive 18kg of vegetation in a day. In experiments gorillas were given over 2000 pairs of foods and they then had to choose between them. Throughout the experiments they demonstrated consistent food choices in high sugar fruits with a high ratio of sugar to fibre, this is even the same using pictures of the fruit!