The grantee of said franchise shall during the term thereof pay to said City a sum annually which shall be equivalent to two percent of the gross annual receipts of said grantee arising from the use, operation or possession of said franchise; provided, however, that such payment shall in no event be less than a sum which shall be equivalent to one percent of the gross annual receipts derived by grantee from the sale of gas within the limits of such City under said franchise and said constitutional franchise.