Hmm well here the thing with GS, so you might better understand, well atleast i actually understand it and im well aware most dont.
Let me first say this, i knew a warrior 4.4kgs that pulled 7kdps pre ICC buff, but he knew his stuff.... so lets to get to the basis of gearscore
GearScore is like your dps/health/healing container, those with low gs like the warrior i was referring to above could theoretically with his gearscore hit lets just say 8kdps, now lets add a warrior same spec, but add 1500 to his GS, the only difference is gear. For your sake lets assume its the same person, the rotation stays the same and he knows perfectly well his class and how to play it. This being said, his highest possible dps would "theoretically" be MUCH higher than the old version of the warrior. This applys for tanking and healing as well just in a different form.
Didn't get it?
Here how about a real life example. You and your friend stand side by side, you both can jump the equal distance off the ground about lets say a ridiculous number 5ft (that measuring feet to floor). You both jump the same way, the only difference is that you are 5' 10" and your friend is 6' 5". Now who can reach higher with a jump (as well as on the ground). If you picked the 5' 10" i suggest you learn some logic or a little bit of math as well (both coincide) The 6' 5" friend will easily out reach you.
Now this usually isn't how it is used, but those who understand the concept use a combo of GS and achievements, tho the combo does not always suffice.
Ultimately there really is not "real" way no matter what you say of checking who is going to have the best dps/gs/or tanking. However if your a programmer and skilled in the field of addons i have a suggestion.
So basically gs is a mediocre means of measuring out (or gambling) on a players dps if used properly, achievements are partial competence. so here is my addon suggestion
SUGGESTION:
We all know what a raid leader wants to see, high dps, and a competent player able to learn the fights quickly, or who already knows the fights, My idea, make an addon that is sort of a communications app, perhaps exports info to a specific website or stores them on your computer for future use. This app would require a Recount/ensidia fails combo, this way a play may link a pug leader the information he or she wants to know "Whats his/her dps and can he/she navigate through boss mechanics properly"