Keep in mind that the GPL has many faces. Depending on the type of GPL license you're using, the rules may be vastly different.
The GPL is known as an infectious license, meaning that any proprietary code that uses GPL code becomes blanketed by the GPL license. This means that, if you did sell your product, you would also need to include the entire source code. The people you sell the product to can modify it, copy it, give it away for free, etc.
If you do sell it, you may be able to profit by offering support that, as the original authors, you are in a better position to do than those who have obtained it and are distributing it themselves.
If the license is LGPL, then as long as the library is only used and is not tightly coupled into the application, then the main body of the proprietary codebase can remain licensed under a proprietary license. However, you still must include the source of the library when you distribute your product, just not your main codebase.
DISCLAIMER: I am not a lawyer. This is my interpretation of the two licenses. It is best to seek advice from a professional specializing in law.