It will be calculated on all resources,not only on gold production as for now. You will lose % of all production, but in treasury they wil be changed in gold by GE(5 gold =10 wood = 2 iron = 5 stone).
Just to be clear, if alliance tax is 10%. Tax will be 10% of Iron, 10% of wood and 10% of Stones (including 10% of Gold), but the non gold resources will be change to gold once it entered the treasury, and not to be taken from the gold production.. And thank you in advance...