First, I’m absolutely not a PHP person, however, I saw similar issues in .NET regarding the PredictionServiceClient. To my knowledge, this is for the older PaLM (2) models and APIs and not relevant for the Gemini API. That client uses the predict method which isn’t offered by the Gemini models. Those models operate on the generateContent method.
Maybe this GitHub sample helps you.
Alternatively, I would suggest you have a look at the REST API and see how you could use it in PHP.