This sounds like a day from hell for Trey Gowdy, wrought with all kinds of peril, even if it's also a day when he could nail down some serious answers and/or expose some serious lies once and for all. But whatever happens, it won't be fun.
Having to spend six-to-eight hours with Hillary would be horrendous no matter what the occasion. In this case, thanks to Kevin McCarthy, she's ostensibly there to testify but her real mission will be to seek out and pounce on the slightest pretext for painting herself as the victim of a political witchhunt. And since the media have at least accepted Hillary's premise to the extent that they too are looking for an excuse to advance the narrative, any question that seems to wreak of politics, even if it doesn't come from Gowdy himself, will be immediately siezed on by Hillary as an excuse to de-legitimize the whole thing.