Given two $n$-bits numbers $a$ and $b$, I am not sure on how to find the runtime of the euclidean algorithm for finding the $\gcd$ of $a,b$. The problem (for me) in here is that apart from the size of $a$ and $b$, I don't feel like I have any other information that will help me to know what is the runtime of the algorithm. I saw somewhere that the number of rounds is $\log_2 a = n$, but, assuming this is correct, I have no intuition as for why it is such?
My question is for an explanation to how much time it takes to calculate the $gcd$ (as a function of $n$)? Is it linear in $n$?