GCD becoming headache , best ways to observe GCD requirement?

I was going through some of the Codeforces and Codechef problem which requires GCD concept , but the thing is that that’s not coming in my head ,
**So please i want a suggestion or problem list that will help me to clear intuition of GCD **

I was wondering how all of a sudden one finds out that GCD is required because I normally see people bending towards GCD when nothing else is coming to mind , so what should I do to achieve flexibility in GCD concepts .
When I somehow figure out that GCD is the key then problem becomes a cakewalk but figuring out is bit challenging I see

problems like
https://www.codechef.com/problems/CIRCHAOS

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