There IS A GAP! Lap-times don't lie. It's simple math, lower means better. So maybe, at Le Mans, the gap isn't that large because Le Mans is, for the most part, a high-speed track. So much so, in fact, that manufacturers build specific configurations for it, while having others for the rest of the tracks. Speaking of other tracks, we've only been comparing LMP1 vs Group C at Le Mans, but that's not the only track in the game. What about more technical courses? What if we have tight or technical sections that will benefit the superior technology, both in aero and hybrid power, LMP1s have? It's not all about Le Mans, they have to balance them everywhere. That makes the BoP for that, even more difficult and ridiculous.
However, I digress because, as others have already pointed out, this ends up being a pointless debate. GT Sport is more about play-ability and competition than it is about raw realism. For raw realism, look elsewhere. Debate over.