I haven't seen a direct comparison, but they have pretty different goals. GLM is meant for working studio monitors, Dirac is meant for home listening. So Dirac can do a lot more correction. GLM is restricted by the fact that it needs to keep latency very low.
All GLM really does is knock low/mid frequency peaks down, and let you set shelf filters to tweak the overall tonality. If you're looking for phase correction, full range correction, or attempting to deal with nulls, then Dirac does those things, but GLM doesn't. You CAN manually alter the individual filters with GLM to do anything that a notch or shelf filter can do, though, more or less.
You can use GLM with a third party sub, just not to cross over or to correct the sub, as GLM doesn't perform crossovers. You will need an external crossover/EQ, either the one built-in to the sub or some other.
The Genelec subs have crossovers and EQ built into them that GLM will control(so you still need to run your audio channels through the sub and then out to the monitors with Genelec subs).
GLM is just a control network for the DSP hardware built into the speakers/subs.