GBTC is a kind of BTC ETF and will give opportunities to "normal" investors to invest in BTC. Yes, I believe it affects BTC price in a positive way since there will be new demand for BTC.

And how do I relate to diff adjustment: Well, normally BTC price follows difficulty as what I understand from mining-price relationship. However, I believe (possible) increase in the price of BTC with this kind of demand outside of BTC world will trigger to miners back to BTC mining (regardless of electricity cost or other more profitable sha256 alt mining) 
Of course , this is just my speculation.