Though the sum total of the basic and D.A of the last drawn wages (based on the definition of " wages" under the Act) is one among the factors in the formula for calculation of gratuity under the Payment of Gratuity Act,1972, we have to note that it is not the wages actually drawn by tthe employee lastly on the date of his termination of employment but the "rate of wages last drawn" as per section 4(2) of the Act. So, the inescapable logical inference is that it is the wage drawable on the date of termination. As per sec.12(1) of the Minimum Wages Act,1948, it is the duty of an employer of any scheduled employment to pay wages to his employees at a rate not less than the minimum wages fixed under the Act.
Therefore, (a) if the last drawn wages is, in general, less than the minimum wages fixed, the employer has to pay gratuity only on the basis of the minimum wages payable.
(b)If the MW fixed by the Govt comprises of components in addition to basic wages, as per the definition of the term "wages" u/s 2(h) of the MW Act,1948 what you've mentioned as "spl. Allowance" will form part of the wages as it can not be bracketed into the excluded items under the definition. If I remember correct that there is a Supreme Court Ruling that the spl allowance if any granted to an employee, in the absence of the specific and explicit mention of its purpose, would form the essential part of the wage for the purpose of indirect monetary benefits of the employment. In effect, if the actual last drawn wages is less than the sum total of basic plus spl.Allow fixed by the Govt, the gratuity shall be payable only at that higher rate.