In this sentence, the word, 'are' is used. This is because 'hamburger' is a countable noun. Too many hamburgers ARE bad for you. If you had an uncountable noun, such as 'sugar', then you would use IS: Too much sugar IS bad for you. Nuno is correct in saying, however, that by adding the present participle 'eating', you would need to use the word IS, i.e. Eating too many hamburgers IS bad for you.

Are. 'Hamburgers' is plural.

Hey, very good question. I am not a native speaker of the language, so I might be wrong. My opinion is: - Eating too many hamburgers IS bad for you. You should use singular because you are referring to the act of eating (singular); I am not sure if you can write that phrase without a context, in my case, eating but it could be with other context. It's commonly used when speaking because it is said on a context and you can easily extrapolate the correct form. Other example: Drinking too many beers in a single night IS really bad for your brain. Anyway, this is how I see it. It would be nice to hear an explanation from a native speaker.