Solution: Factorize $ 210 = 2 \cdot 3 \cdot 5 \cdot 7 $ and $ 294 = 2 \cdot 3 \cdot 7^2 $. The GCD is the product of the smallest powers of common primes: $ 2^1 \cdot 3^1 \cdot 7^1 = 42 $. oxed42