By gcd, I think you mean gcf ( Greatest Common Factor).
To find the gcf find all the factors of each number:
Factors of 20: 1, 2, 4,5 ,10 , 20
Factors of 56: 1, 2, 4, 7, 8, 14, 28, 56
The largest common factor is 4.
LCM = Least Common Multiple
This is the smallest number that both numbers divide into evenly
Find the prime factors of each number:
Prime factors of 20: 2 * 2 * 5
Prime factors of 56: 2 * 2 * 2 * 7
To find the LCM, multiply all prime factors the most number of times they occur.
In the prime factor of 56, 2 appears 3 times and 7 appears once.
In the prime factor of 20 5 appears once.
LCM = 2 * 2 *2 * 7 * 5 = 280
