As I know, softwares licensed under the GNU GPL is totally free and not allowed for sale. However, today I found this program called Minitube and it is sold even though it is licensed under GPLv3. Does the original author has a permission to sell his own software, even if he releases it under the GPLv3?
You are wrong. You can sell code under GPL. And you don't have to be the owner.
But you also have to provide/publish to your customer or user source code modification. (And your customers are allowed to redistribute the code free of charge. This is why most GPL code is free as in beer.)
One of the main idea behind the GPL is that the user must be able to modify or audit software. But the licence itself does not refrain you to sell the software.
See this page from the FSF on selling GPL software.
Yes, the original author is not bound by the terms of the GPL. The GPL is a License granted to any person who has a copy of the program. If you don't have a copy of the program, you are not entitled to the source code. The original author is also allowed to release the software under as many licenses as they wish. Once a piece of software has multiple authors, they must all agree to any licensing changes.
The GPL is a copyright license. Such licenses only apply to people other than the copyright owner. The original author of the code is free to do with it as he wishes.
This makes dual-licensing possible, typically done by giving away GPL-licensed source while selling the same code to closed-source developers under a different license. That's how Trolltech made money from Qt. It is GPL-licensed, but before Nokia bought Trolltech, a developer seat for the proprietary version of Qt was quite expensive, a thousand dollars I think.
As pointed out above, despite the fact that the GPL is a Free Software license, it doesn't mean that you are not permitted to charge money for it. "Free" in this case refers to the rights passed on to those who obtain GPL-licensed code. You can charge anything you want for GPL code, you just have to abide by the terms of the license.
Lots of companies make vast sums of money selling GPL code: Google with Android, IBM with Linux for their RS-6000 servers, Red Hat with Red Hat Enterprise Linux. But all of their source that is linked with GPL source has to be GPL-licensed itself, and has to be made available to those who obtain copies of the binary.
Note that you don't have to actually distribute the source. Lots of people think that one has to post one's source online to comply with the GPL. That's not actually the case. Neither is it sufficient to post the code online: Richard Stallman pointed out a while back that those without an Internet connection would not be able to obtain your source if that was the only way you distributed.
To comply with the GPL, if I remember correctly, you must make a written offer to provide the source, and you have to actually provide the source when you are asked for it. You can charge a "reasonable" amount of money for the source, to cover the costs of media duplication, shipping and handling and so on. Even the Free Software Foundation sells its own source code on physical media, despite the fact that it is all available online.