No, they mean the same thing. They both describe an unreal present. The second one is a bit less formal because of 'gonna'.

Practically no difference. Only very subtle nuance of difference. The first is suitable in a formal context. The second is more slang. Also the second sentence is more likely to refer to the immediate instance of coming (you might say it while at a party, for example) while the first sentence is slightly more likely to refer to a time in the distance past. But both can be used in either context - it's only a subtle hint of differentiation.