Ok, firstly i'm a known pugger on Kalecgos and have tons of experience pugging TOC10/25, TOGC, Ulduar 10/25, OS3, VOA10/25 etc...
Gearscore shouldn't be thought of as an absolute. It's just for generally checking people.

For example if your doing TOC25, but someone registers as gearscore 2500, their obviously wearing blues or greens not even ready for it. Either that or they are not wearing their full gear. Check if their naked or wearing an alt spec gear set.

If they are wearing PVP gear, ignore their gearscore and deduct a few hundred points off it. PVP gear is known to inflate gearscores and it hardly useful for pve encounters. I usually tend not to invite people with PVP gear.
After doing gearscore, check what gear they have. Their gear tells you a story where they have raided. For example if they got Heart of Iron, means they've killed Ignis in Ulduar 25 before.... If they have the Crusader Glory sword itemlevel 245, means that they have killed Anub in TOC25.... basically you want people who from their gear seem to have quite some raiding experience, otherwise how else would they get gear from these bosses??? (unless they bought their accounts which is usually rare)

for TOC10 and 25 i usually bring gearscore 4600+. For TOC10 the lowest you can get is probably gearscore 4200-4400
Tanks should have the highest gear score, followed by DPS. Healers can have the minimum gearscore. They should also be skilled healers :/
Gearscore just tells you generally how well geared they are, but it doesn't tell you if they are skilled. Maybe if they were TOGC and Ulduar hardmode gear it is 90% likely though they are very skilled.