Hi
You will need to survey the area first as the extent of the gum will undoubtedly vary from one part to another.
Figure out how long it will take you and then charge accordingly.
The method you use will obviously determine how long it takes you and the results you will get, also, you might want to sell the customer the service of having the whole area cleaned too, otherwise it will look patchy if you just remove the gum.
Hope this helps
Andy