I think you should look at the whole picture moon. ERA-/+ is a great stat but it only adjusts league/park flaws which is not a minor thing, but it doesn't isolate Pitcher's abilities and luck like FIP does, it doesn't normalize HRs like xFIP does, and it doesn't consider BABIP like SIERA does.
Said that, if you put all the later in a mixer, I don't think is a good idea land Gray.