While you may be able to use Ruby, you won't be able to use RGSS, since there are hidden classes and other items not stored within user access. However, if you have sufficient scripting knowledge, it would not be impossible for you to write replacements for those classes (I know Sephy and Trick did so).
However, there really is no benefit to RGSS outside of RM. Fret not- If you know RGSS, Ruby isn't hard.