I've also used Gearbox many times for guitars, as well as Guitar
Rig. I always found that there's a certain brute harshness missing from heavy guitars
(like you say - especially the top end) that I couldn't capture via re-amping. Believe it
or not, I've had better results miking a Line6 Spider IV than using Line 6's gearbox!
All this changed when I started getting more 'in depth'. Try looking into
'impulse responses' - they are a more convincing/acurate 'modelling' of the tonal
characteristics of an cabinet/mic combination in a room - http://www.redwirez.com
do some free
ones and some fantastic products.
Ultimately , I settled on using a 're-amp'
VST by TH2 called 'Overloud'. It's awesome for distored rock/metal guitars and it also
allows you to use custom 'impulse reponses' - this is equivelant to using an 'amp' from
gearbox with a much better 'cab' simulation by someone else.
working on my band's new EP, a rough mix with the above technique can be heard here:
The guitars have no EQ on them - only the TH2 VST and a limiter.