GCD and Integer Relationships

Let $$a$$ be an integer that is a multiple of 12, and let $$b$$ be an integer that is 5 less than a multiple of 9. Specifically, $$b = 9k - 5$$ for some integer $$k$$. Consider the following two quantities:

Quantity A: The sum $$a + b$$

Quantity B: The greatest common divisor (GCD) of $$a$$ and $$b$$.