Disclaimer: I'm not a lawyer
(section 2a)
You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
This indeed confirms that you may charge a fee in some cases. e.g. warranty protection.Since "you" refers to any licensee, this means anyone who licenses the program may do so. Any licensee who wishes to become a "warranty seller" can.
(section 2b, emphasis added)
You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.
This means that every licensee must not charge to license the program. However, you may still charge for warranties, etc. Provided you also offer a version of the program without that warranty.As for the physical transfer fee, that might be an issue. However, since the licensee can freely distribute it, only one person would be required to pay that fee - then that person could redistribute it to others for free.
This section might be problematic:
These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.
If I understand this correctly, that means that artwork etc. may not be GPLed, but when distributed as a whole, then it must be GPLed.
This means that a game that's distributed "as a whole" must include the artwork as GPL.
If you can't license the artwork as GPL, then the GPLed portions of the game can't be distributed as a whole either. For this situation, dual-licensing seems to be the only option.
My understanding of what a "whole" is is kind of fuzzy, though.