I.e. foreigners travelling with a tour agency based in Georgia. In their quotations they mention explcitely that the quote does not include tips for the drivers.
How much is it customary to pay? i.e. not by locals but by foreigners.
While I've never been to georgia, when I was touring eastern europe with friends, we usually tipped all our drivers 5% - 10%. Depending on where you're travelling from, if you can afford to let go of a few coins then whether it's customary or not, it's a good idea.
I got an answer from the tour operator:
Though we pay a good salary to our staff, clients usually tip them for their good job. Generally on a tour for 2 persons they tip minimum US$ 20 - $25 total per day for the guide and about $15 - $20 total per day for the driver. However, it is upon you to add more or pay less.
It is nevertheless quite unclear.