The thing about the LMP1 class is that it is, to woefully over-summarize the issue, an unlimited class. Last I checked, there were few-to-no restrictions aside from the safety aspects, and maybe more importantly is un-capped. LMP2 I believe has a cost cap as well as actual restrictions.
As for GT, does anyone remember the ALMS GT1 class? More to the point, what happened to it? For those who dont, the class had been largely dominated by Corvette Racing for years, but other were still fighting. Around the time the C6.R was introduced, Aston Martin came in with the DB9R, and soon after it was either Corvette or Aston Martin winning, no one else had a shot. Teams migrated to GT2, and eventually GT1 was Corvette vs. Aston Martin, and as AM was throwing in the towel, Corvette re-engineered the C6.R to meet GT2 regulations.
Now note that I AM NOT AN EXPERT! This is how I remember it all.