i'm not sure how the gearscore matches the instance levels, but think of it this way:

the 70-79 normal instances drop iLevel 138-183 items
the level 80 "normal" instance, drop iLevel 187 items
the level 80 "heroic" instances drop iLevel 200 items / iLevel 200 epics
the Icecrown normal instances drop iLevel 219 epics
the Icecrown 'heroic' instances drop iLevel 232 epics

have a look at your gear, and check what the average iLevel of your toons is.
if it's around 187, then it's time to move towards heroics,
if it's around 165 then, keep on working on normal level 80 instances
and so on :-)