Here's a rather odd method of determining this....but it works!!! It's called the "Sieve of Eratosthenes"
First take the smaller given number and subtract it from the larger......so we have......
20 - 8 = 12 ........Now, does 12 divide both 8 and 20??? Nope. Now, compare what we subtracted (12) and the result we obtained (8) and, again, subtract the smaller from the larger.
So we have 12 - 8 = 4........Now, does 4 divide 8 and 12?? Yep. And that's our GCF...!!!
Why does this work?? Because if a number divides two subtracted quantities, it also divides their sum. So 4 divides 8 and it also divides 8 + 12 = 20 !!!
Note, if we use this process on two numbers and we end up with a "1," the numbers are relatively prime to each other. That makes sense......only one non-zero number divides two relatively prime numers....namely, 1 !!