Small claims is for money damages, you won't be able to discover the identity of a Google user through that process. Additionally, Google is very protective of its users' identities and almost always requires a court order to turn over such information.
You should contact an attorney if you are seriously considering proceeding with this matter who will be able to advise you about filing a petition for pre-action discovery. Through this petition you can uncover the identity of the poster if you can establish to the court that you have a meritorious cause of action and the information you're seeking is necessary to bring that action.
The above answer is not "legal advice" as specified under any pertinent rules governing the Professional Responsibilities of Lawyers and should not be relied upon. An attorney-client relationship has not been established by virtue of this correspondence. Legal issues are often complex and involve local laws and facts which may not be effectively communicated without a complete consultation.
I agree with the previous responses. This is serious libel / slander and you should consider talking to an attorney about pursuing this case. A major consideration is damages: can you prove that your establishment has been damaged (e.g. lost clientele / customer base, profits, etc.) I would only recommend pursuing this beyond small claims if it makes economic sense.
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