Unless they IP ban the whole of the EU, they will still be subject to the GDPR once any EU citizen accesses their site. (Which, agreeing with AmericanChopper, is a purely theoretical thing - until the moment where YC wants to conduct any kind of business in the EU)
If they raise money from any limited partners in the EU it’s pretty easy to make a case they have an EU business. The physical location is irrelevant - that’s the whole point of the GDPR.