To get an idea of the probability of successful dispel as a factor how many extra gems the dispelling mage is using I simulated things for d6.
The attached graph is for d6.
I think that the graph for oed6 would be similar except that it is asymptotic to 1.
Therefore I think that if the caster will add 3-4 gems extra to what the GE caster used he would have a good chance for dispelling (~80%-90%).
more then that would give diminishing addition to the chance of dispel.
Well probably that's an obvious conclusion, but I find it useful nonetheless
