There are differences in meaning in some contexts, but in a general context they are the same. Basically גבר means man while איש is the masculine form for person (i.e. man, but this subtlety is the basis for the contextual differences). The difference I think is most important is in the plural form:
- גברים = men
- אנשים = people
- אישים = [notable] people (very narrow context)
In the context of this sentence there is also a subtle difference:
- אתה איש = You are a person (masculine). This does not emphasize the gender – and you probably won't say this in real life, it sounds strange.
- אתה גבר = You are a man – often used to emphasize manliness, but can also be sarcastic, i.e. "some man you are".