The newest "fad" in pugs and even in guild recruiting is "gear score". So, to start, I'm going to list reasons why gear score fails hard. In this blog I'm referring to gear scores obtained either via the addon, and/or via websites such as... well actually, I'm not going to list the websites, because we don't really need any more mouth breathers going to them.
1. Gear score says that my warlock's destro PVP spec is better and gives a higher score than my warlock's PVE spec.
2. If I use my resto gear on my druid, and spec feral, gear score still gives me the same score as I had as resto. However, my dps would be terrible in my resto gear.
3. Gear score says that my warlock has a higher gear score in PVP gear where I have less spell power, and I'm not hit capped, and I have less crit.
4. Gear score can't tell you if someone has gemmed appropriately, or enchanted appropriately. Example, the 15 spirit to chest enchant is the best PVE enchant for warlocks, but gear score says it sucks and that +10 stats is better.
5. Every pug I've joined where the leader is looking at gear score typically fails, or takes many wipes to down something. Onyxia is a great example of this. "Hey guys, everyone has great gear scores so this should be easy". Meanwhile we wipe 20 times because no one can understand the concept of "deep breaths".
Gear score is a crutch that only perpetuates the terrible. If you have to rely on an inaccurate arbitrary number generated by a website or addon to run your pug, then you have no business running a pug. Gear is ridiculously easy to get in warcraft these days. As someone leading a pug, you need to learn how to sift through the morons and the bad players. Focus more on enchants and gemming. Do their enchants and gemming choices make sense? How about their itemization and gearing selections? Did they prioritize the right stats? How does their spec look? If you know what you're doing, it'll take a few seconds looking at each person you invite to see these things. And yes, this means you need to know other classes besides the one you play. If you don't, then you have no business running a pug.
If I knew how to write addons, I'd try to create one that focused on gemming choices, enchant choices, and itemization for each class, based upon ratios of stats. Using these factors will assist you in weeding out drooling idiots who don't know how to get out of fire. I will gladly take someone who has less gear, but has the right gems, enchants, etc, than someone with more gear who looks like they threw darts to decide what gems to buy.