As a learner, don't say "gotta" or "gonna" under any circumstances whatsoever. Forget these slurring contractions exist. They are casual to the point of illiteracy, entirely inappropriate in writing, and when combined with an unavoidable accent sound terrible to native ears. If you choose to use them, be prepared for unnecessary rudeness and the loss of career or business opportunities. I'm sorry to have to say it, but that's just how it is.
The proofreading rule -- probably artificial, but used nevertheless -- is that nominal or-phrases are considered singular or plural depending on the noun, pronoun, or numeral closest to the verb. Therefore,
I am not sure whether you or Richard is going to do it.
I am not sure whether Richard or you are going to do it.
I agree that "are" sounds more natural here. Therefore, if you don't want to rephrase, choose the option with "Richard or you". There is no difference in meaning or emphasis between the two choices.