Garmim elevation gains - it depends. If you read all the forums over at motionbased and tons of other websites, the barometic measurements can be somewhat high after you've used the gravity-fixer thingie in the training center or MB, and the jury seems to be mixed as to which does a better job. Overall, it appears from what I've read that the totals can be ok to somewhat high, but at least somewhere in the ballpark. I've only been to Boise a couple times but I don't remember huge mtns. Definitely some climbing and rolling hills, but nothing gigantic and long like the Sierras or the Cascades.
If you have the Edge or Forerunner which only uses GPS elevation, your totals will be really, really high.
Barometric altimeters are the most accurate but it even depends on the particular instrument itself.
Frends know gud humors when dey is hear it. ~ Da Crockydiles of ZZE.