You are confusing GaussView and Gaussian. Gaussian is the software that really calculates molecular properties. GaussView is just an interface that helps to build molecules and create Gaussian inputs. It should not matter what software you install first, but personally, I would install Gaussian first and GaussView second.
I agree with Suarez, the latest version is GaussView 6 ( https://gaussian.com/gaussview6/ ). and there is no installation order. Both programs are independent. You can even install one without the other. or for example install Gaussian 16 with version 5 of GaussView.