a 20 def gem will give you about 4 to your defense skill. The avoidance gain will vary on how far into the DRs you are, but I think on a warrior you get as much as .12% or more on all stats it affects from a 20 def gem. Is this what you're looking for?
If you're referring to some addon that tells you how you should gear I have nothing for you. Rating Buster has served me too well to try another addon to compare gear and gems.