A leading digit is the digit/number at the beggining of a decimal number or regular number and is the first number to the left.
GCF of 253 is 253.There cannot be a greatest common factor of just one number. To be common there need to be at least two numbers. If you find all the factors of two or more numbers, and you find some factors are the same ("common"), then the largest of those common factors is the Greatest Common Factor.