I'm honestly confused by how difficult this seems to be to people - while the iPhone X isn't a 4K screen, it is approaching 2.5-3k and how do games look on it? How does Fornite look on it? Now remember, this iPhone X GPU is targeted for mobile applications, a very limited TDP. Even if we didn't consider, this speculation would use a GPU two generations out, on a proposed manufacturing process half the size of today's chips - why couldn't Apple make a GPU that's 4x or 6x the size of their current one?
We've seen cost-breakdowns of the iPhone X, the CPU and GPU combined is something like $30 when at their production scale - just use common sense and rationale thought to work out how feasible this is, Jesus...