There has only ever been one issue that I'm aware of where an App was pulled,
for GPL violation, and it came out that the reasons were political.
The Copyright holder complained about the App in question conflicting with GPL
because he was working for another company at the time with conflicting
interests with Apple and/or the App itself.
If the GPL code you wish to use has a single author, you can write him and see
if he will create a dual license for you. This has been done before, and is likely
to happen if your App is also going to be free, as intended by the author.
There is plenty of GPL code in the App store including a free App of my own.
Copyright holders are the only people who can take action against an offending
App, but Apple may react to any complaint to avoid any trouble.
In my case, I contacted as many authors as possible, (since out of 70 or so
contributors, some may no longer be with us), and all were happy for their
contributions to be released in the App Store for free to the end user.
I am not a lawyer, and none of this constitutes legal advice,
but if you have your heart set on the game, I would simply contact the graphic artist,
and I'm sure you'll get the same result for the graphics too.
You can do your best to comply with GPL at the same time as the license created for you
by making it free, and providing source code for your modifications
on the support website that is included in the App Store description.
Apple have not frowned on me releasing source code for my App Store Apps
whether it be GPL or Public Domain.