This が is used to introduce a new thing into the discourse. It's one of the most basic functions of が, and I believe you have learned it before. It roughly corresponds to "a" as in "a good restaurant". See: What's the difference between wa (は) and ga (が)?
Would は be indicating a contrast?
The sentence in question has nothing to do with a contrast. It's just a plain sentence that means "There is a good restaurant". If you used は, that would turn the sentence into a contrastive sentence.
(Although there is no shoe store / I know you're looking for a bar, but) There is at least a good restaurant in this building.
This makes sense only in a special context.
This leads me to a general question about intransitive verbs: it seems that intransitive verbs almost always have が before them so why is が used over は in these cases?
Transitivity of the verb has nothing to do with the choice between が/は, so you should forget that observation. We can safely say 彼は走った "He ran", for example. (If you saw が used for something that has already been introduced into the discourse, it may be related to neutral description-ga. But it has nothing to do with the original sentence in your question.)