Learn Spanish (Latin America): El gato gris come pescado.

In Spanish, describing words (adjectives) usually go after the thing they describe. So, instead of saying "gray cat" like we do in English, you say "cat gray" (gato gris).

I thought the word for fish was pez. Why does this sentence use pescado?

Pez is used for a live animal swimming in the water. Once the fish is caught and becomes food, the word changes to pescado. Since the cat is eating it here, we use pescado.

Why isn't there a word like el or un before pescado?

Just like in English, when you are talking about eating a food in general, you don't always need an article (like "the" or "a") in front of it. Come pescado simply means "eats fish". If we said come un pescado, it would mean the cat eats one whole, single fish.

Why does the verb comer change to come here?

Comer means "to eat". Because we are talking about what el gato (a "he" or "it") does, we drop the -er ending and add -e to make come.