Answer: Gary has made mistake in taking the greatest common factor i.e.
he takes 3 instead of 6.
Explanation:
Since we have given that

He has made mistake.
As he wants to apply the greatest common factor to determine the expression.
But we can see that there is greatest common factor is '6'.
So, using distributive property:
6(11+6)
Gary has made mistake in taking the greatest common factor i.e.
he takes 3 instead of 6.