Can someone pls. guide me or give me the list of important Maths Algorithm used in Competitive Programming . I searched it on gfg but the list is HUGE !!! . Can’t figure out what to study what not to .

I assume you’re talking about this list. I’m not going to pick out specific articles, but give you a general rule of thumb for doing so. You should really only do things that seem ‘general enough’.

GCD of an array? Yes.
First N natural can be divided into two sets with given difference and co-prime sums? No.

I say this because the point of doing competitive programming is to become better at solving problems knowing only some basic concepts. Not to have knowledge of solving a specific set of problems. You should be able to solve any problem in a given section of that blog, if you do the first few.