There used to be an RStudio ebuild in the default Gentoo overlay. However, it got removed because it didn't work. See this URL: https://bugs.gentoo.org/855197. I think there should be a new RStudio ebuild because it's been four months and we still do not have a working RStudio ebuild. RStudio is a very popular piece of software so I am sure I am not the only one who would like to see this.
*** This bug has been marked as a duplicate of bug 766294 ***