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.
Since 36 is a factor of 108, all of its factors are common.
common factor of 36 and 52 = 2
The greatest common factor of 27 and 36 is 9.
The greatest common factor of 147 and 36 is 3.
The greatest common factor of 36 and 57 is 3.
The greatest common factor of 17 and 36 is 1
The greatest common factor of 36 and 400 is 4.
The highest common factor of 40 and 36 is 4
The greatest common factor of 1624 , 36 = 4
The greatest common factor of 36 , 58 = 2
The greatest common factor of 79 , 36 = 1
4 is the greatest common factor of 36 and 140.