No point in trading Grubauer at this juncture. He fills a distinct role on a contending team. The Capitals need a back-up goaltender, and Grubauer fills that and offers far more upside should Holtby get hurt than any goaltenders left on the market.
The concept of trading Grubauer can be broached again before the expansion draft (as there's currently a pretty decent chance Grubauer could be picked up by Vegas). However, the return would have to be much higher than what's being discussed here. While the buyers market for goaltenders leading into the expansion draft will suppress prices some, look at the returns other similar goaltenders have fetched in the past few years. Based on the returns for guys like Martin Jones, Cory Schneider, and Frederik Andersen, Grubauer could likely command a late 1st on a typical trade market. In the lead-up to the expansion draft prices will fall a bit, but the Capitals shouldn't consider moving him for anything less than a 2nd. Otherwise it makes sense to hang onto him. Even if they end up losing Grubauer for nothing to Vegas, that still has value in terms of preventing Vegas from acquiring other exposed assets (such as Orlov).