Hello,
No, It is not right. I think American English is quite different from British English.
Here in the United States, going to, is used for the near future, and tomorrow is not considered in the near future.
So you either have to say:
I'm gonna go fishing right now or I'm gonna go fishing in a few hours.
Or if you want to use the adverb "Tomorrow", You gotta say: I'm will go fishing tomorrow.