Day traders / high frequency traders are super reactionary. As an example, AMD also inexplicably dipped this morning, despite the nasdaq being up, presumably because they also do GPUs. But, I’m pretty confident that nvidia’s valuation is based on their growing strength in hpc/ data center, and this being the case, this momentary downward blip based on consumer product performance will correct itself shortly.
Ultimately, datacenter, embedded (e.g. infotainment centers for cars), and IOT is where the money is at; not in gaming. All you have to do is look at NVIDIA's website; look how far buried the Geforce stuff is in the menus compared to basically everything else. That should tell you where their priorities are these days. There was a time when the NVIDIA website was about nothing but gaming graphics.
AMD's valuation is based on Intel being a rudderless ship allowing them to come in and claim market share. Brian Krzanich was an unmitigated disaster for Intel; under his reign they lost their manufacturing lead for probably one of the first times ever (at one time, Intel was years ahead of the competition in terms of manufacturing technology). Couple that with the fact that they haven't significantly changed their processor architecture since at least Haswell, AMD coming out with Ryzen, and the Spectre/Meltdown stuff and it has not been a good year for Intel. When Intel announces that they are hoping to keep AMD marketshare "under 20%" in a market where they have 99% marketshare currently (servers), you know things aren't good for them because they are facing a war on two fronts; AMD on the high performance side and ARM coming at them from the other end for mobile/power efficient chips.