Need help with a gun mod (NEW PROBLEM)

Hello, i am working on a gun mod called Simple yet Powerful, and im stuck on making skins and attachments for the guns, i really want it to be in the first version but i dont know how to change an item's texture/model dynamically, if anyone could help, i'd really appreciate it, also please explain it in a simple way, i dont know java. 

-- that is the part somebody helped me solve. but not entirely

see, it would work in normal items, but im using geckolib animated item to do this, so can anyone still help, Or do i need to code?

PS: Sorry for bad english, im from Poland.