Is it wajib to say the Shahada whilst performing ghusl, for example you wash your head and neck and whilst doing that you say the Shahada and same thing applies to the right and left side of the body? Or can you just do the action of ghusl and not say anything?

No you don’t have to say the Shahada. You just make the intention in your mind that the ghusl is for Allah and you do it.