The Winkelvoss twins are in Bitcoin around four years. So far, they have not had any great success although they have a good amount of starting capital and some venture capital. I guess, these guys to not have any talent for running businesses or startups.
Gemini is not that big that it will has much impact. Whoever is interested in Eth could buy it on different exchange like Kraken or Bitfinex.
So, my guess, there will be no impact.
It can result in a slight increase but like stated the volume on gemini is not very large right now. Having eth available to more investors is a good thin g btw.
