It is true that the GCF of any two even numbers is always even. GCF refers to the greatest common factor that two or more numbers have, and obviously it is going to be an even number. Take 18, 30, and 42 as your example - their GCF will be 6, which is an even number, as are those three numbers. GCF of odd numbers will be odd, and GCF of an odd and even number is also always even.