And then what? If you say, "actually sir, that's not true" - he'll just call you nasty and try to have you barred from the press pool. And nothing else will happen. The lie will become a new talking point. Online liberals will guffaw at the absurdity, but most people won't even see it.
I think the real journalists asking Trump questions during press gaggles are wasting their time. They should just let the right wing hacks ask the questions and record what he says - for the record - and then just fact check him. That's all that real White House journalism can be these days.