Pennsylvania had a redistricting plan which was a partisan gerrymander. 18 districts, and the GOP had 13 seats. That's 72% of the seats. They don't have 72% of the vote. That's a bias. A few years ago, I wrote a paper called "Population Equality and the Imposition of Risk on Partisan Gerrymandering." Basically, because every district has to have the same number of people and you can only shift people around so much, there is only so far you can push a partisan gerrymander without just shooting yourself in the foot because taking maximum partisan advantage requires drawing districts where you have slim partisan advantages in as many districts as possible. Those advantages can go away if the partisan winds shift. That's risky.
In my assessment, Pennsylvania went about as far as you can go without just getting stupid and imposing what Bernard Grofman and Tom Brunell call a "dummymander." Pennsylvania's state constitution prohibited partisan gerrymanders. The US Supreme Court didn't block the Pennsylvania courts from ordering a new plan, so... new plan.
The US Supreme Court did not rule, in the context of this case, that a partisan gerrymander is unconstitutional at the federal level. That's what Gill v. Whitford is about. So, does this matter?
Yes. Or... maybe. It means they are open to thinking about partisan gerrymanders and ruling on them.
In the past, SCOTUS has said, basically, that partisan gerrymanders might be unconstitutional, but they didn't have a standard. Gill v. Whitford is making its way through the system because there is a standard proposed (truth in advertising, I vaguely, kinda know Eric McGhee, who is the guy proposing the standard, because we were in grad school together). If the SCOTUS isn't just laughing the Pennsylvania case out of court, then...
Oh, I hate this tea-leaf-reading crap. Congress is much easier. 435 House members versus 9 Justices. Is it any wonder why I prefer the former?