The GCD (also called GCF) of two things is the biggest thing that goes into both evenly. So, for two terms, it's the biggest term that goes into both evenly. Its coefficient (that's the number in front) will be the GCD of the coefficients of the two terms you're given. The exponent on each variable will be the lesser of its exponents in the two terms you're given. (If some variable doesn't exist in both terms you're given, then is doesn't appear in your answer either.)
For example, suppose you want to find the GCD of b and a4b2c. The coefficients of both these terms are 1 (remember, if there's nothing in front, it's the same as if there's a ‘1’), so the coefficient of the answer is also 1. Since a and c don't exist in the first term we were given, they don't appear in the answer. The letter that does appear is b. Its exponent in the first term is 1 (remember, having no exponent is the same as having exponent 1), and its exponent in the second term is 2. The lesser of these is 1, so the answer is 1b1. This is the same as just b, so that's our answer.
Remember, having no coefficient (or exponent) is the same as having a coefficient (or exponent) of 1 (not 0).