Because non-gibberish doesn't mean "the best/most common way to communicate". If you speak in English like Yoda, the character from Star Wars, you're perfectly intelligible -- hence, you're not speaking gibberish -- even though (almost) no-one speaks that way in real life.

Gibberish means nonsense. If most people understand you, it's not nonsense.

But how do you know most people understand you if you're not speaking like most people speak?

You're right: I do not know for sure that most people understand Google Translate's translations (with more or less effort). I assume they do because I do, and because other people I know also do.