I suspect the mavity bit is either a tease for an upcoming plot point about changing history or just a running gag that will get called back later.

They didn't know Newton misheard them. Couldn't have. Donna said it as 'mavity', and the Doctor gave her a momentary odd look. But then when he said it as 'gravity', she didn't know what he meant and he corrected himself to 'mavity.'

What that means is that history was changed, and the word is now 'mavity' and always has been. Nevermind Latin etymology and the fact that of course Newton didn't actually invent the word but only popularized this specific use for it.

The point was that the change affected Donna but not the Doctor, because he's a Time Lord. Whether it's just a joke or a hint at something to come remains to be seen.