RGSS is the set of scripts that make RMXP what it is. It is basically a "framework" of a basic game, that can be built upon. They are written in Ruby, and use Ruby to execute.
And, no, you don't need to learn Ruby first. By learning to write code for RMXP, you are in fact learning Ruby. The actual Ruby language is much more expansive than what it used in RMXP, so in my opinion, it is actually easier to start with RGSS, and then built upon that knowledge to learn Ruby, though they each will allow you to learn the other.