It's a play on words. Donald is the baby's great-grandfather. Donald is saying he's already earned the right to be called a great grandfather because he's raised his grandkids into adulthood in their father's absence, which makes him great.

They are talking about the discussion over what to name the baby. I am a native speaker, and I am not sure exactly what he is referring to. But it is a good movie!

I watched the video and i think that the subtitles are incorrect. When he says "earned" it sounds more like "learned". It makes more sense if "donald has already learned the great part of it". The "great part of it" meaning taking care of the child, as seen in the video. Happy to help!