Yeah I think that how much two countries names are similar should be measured by percent, rather than number of letters that need to be changed/added. If you had two hypothetical countries, called 'Bo' and 'Fa', they would end up on this list having only two letters needed to be changed, but really the words would have no letters in common at all. The percent based scale also values longer countries, as having 6 of your 9 letters align (three letters off) would be the same as having 2 out of 3 letters aligning (one letter off).
From Zambia to the answer you should have
Change 1, Add 1 in this order (as for Gambia)
I don't know if I'm right but I assumed you made the transformation in order.