what to clean gearbox of mixer with?

I don't think you do need to clear out the old grease - just pack in a bit more.

But if you really do want to make the gearbox as-new then wipe out the grease with paper towels and use an old toothbrush and soapy water to get it squeaky clean.

Petrol and other solvents will work but they're a bit excessive and dreadfully smelly - the petrol fumes will linger unless you wash it clean with hot soapy water - which is where I came in.

Good Luck...
May 2016