The guys at Croydon Racing Developments run the Brembo GT kit on their R34 GTR with 18" TE37s.
The LMGT4 is a very similar designed rim so clearance wont be an issue.
Their car has (im pretty sure) 8 pot fronts with 376 or 380mm rotors and they sit well.
I have a set of Alcon 6pot calipers from a Daytona LeMans Prototype on a 375mm rotor. Dummy fitted up, they just squeeze inside a 17x9.5 +12 TE37 with about 5mm of clearance radial wise. Plenty of caliper clearance behind the spokes.
The alcons are also a larger body caliper compared to the Brembo GT, so that coupled with a smaller diameter, eg 365mm, you should have no problems fitting it into an 18" LMGT4.
Also, if you ask nicely, Im pretty sure Matt (God_speed) on here is running the GT kit under 18" LMGT4s. I could be wrong.
Hope that helps.