There cannot be a greatest common factor (GCF) 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.
The greatest common factor is 28.
The greatest common factor of 154 , 28 = 14
The greatest common factor of 28 and 55 is 1
The greatest common factor of 2702 , 28 = 14
The greatest common factor of 7 and 28 is 7.
The greatest common factor of 28 and 20 is 4.
The Greatest Common Factor of 28, 48 is 4.
The greatest common factor of 48 and 28 is 4.
The greatest common factor of 16 and 28 is 4
The greatest common factor of 28 and 120 is 4.
The greatest common factor of 28 and 250 is 2.
The greatest, and only, common factor of 57 (=3x19) and 28 (=22x7) is 1.