There is nothing wrong with Justice Gorsuch speaking at the luncheon despite its venue. Any relationship between a single event and the Emoluments Clause litigation is far too tenuous to implicate Gorsuch’s impartiality. The justice is obtaining no benefit from President Trump, and the president is obtaining at most a de minimis benefit – and really, not even that – from Gorsuch’s appearance at his hotel.
It is unfortunately true that President Trump has abandoned previous conflict of interest standards that were thought to restrain presidents from cashing in before leaving office, but his continuing ownership of the hotel does not violate the federal conflict of interest statute. Thus, Gorsuch’s lunchtime talk is neither an endorsement nor an acceptance of Trump’s decision, unless one thinks that the very existence of the hotel should be boycotted (which of course, would also be a statement about its legitimacy).