Gemma is their open-source series of models. Gemini is the propertierary ones. Gemini models are bigger and better. But Gemma are pretty good too.

open-weights, not open-source (sorry to be that one but open source in this case would mean you can build it yourself from provided "source", which you can't, because it's not provided)

And even "open-weights" is generous, as they're released under a proprietary license with usage restrictions, not an open-source license.

"Weights available"