The glue code and GUI code are original works by their author. When considered in isolation from any other code, they likely not derivative works under copyright law. They don't include any of the GPL-license code; they merely use its API or invoke it as a separate process.
The author is certainly free to dual-license the work. The fact that he is distributing the work as part of a larger GPL work merely means that he must license the work in a GPL-compatible way whenever he does so.
The recent rulings in Oracle v. Google in the U.S. about the copyrightability of an API suggest that possibly the use of the GPL software's API could make it a derivative work, but considering that Google's complete re-implementation of the Java API still qualified as fair use, some tiny use (not even re-implementation) of some software's API is not likely to cause the creation of a derivative work of that software.
Since the glue code is an original work of authorship, the author may license it however he pleases. Obviously, when he chooses to distribute a GPL-licensed work in combination with his own work, he must abide by section 2(b) of the GPLv2 and release the whole combination under the GPL:
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.
But this requirement is immediately qualified by:
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.
So long as the glue code and GUI are "not derived from the Program, and can be reasonably considered independent and separate works in themselves" then the author may license them individually without regard to the GPL, whenever no actual GPL code is involved.
When GPL code is involved, the author must include source code for his component, if the combination forms a derivative work under copyright law. If the combination is not a derivative (i.e., the new work "uses" the GPL work "at arm's length" but does not become part of the GPL work), then the GPL requirements do not apply to the new work. The GPL FAQ has a good overview of the FSF's position on the issue, which is quoted in part below (but I recommend you read the whole thing!):
What is the difference between an "aggregate" and other kinds of "modified versions"?
An "aggregate" consists of a number of separate programs, distributed together on the same CD-ROM or other media. The GPL permits you to create and distribute an aggregate, even when the licenses of the other software are non-free or GPL-incompatible. [...]
Where's the line between two separate programs, and one program with two parts? This is a legal question, which ultimately judges will decide. [...]
If the modules are included in the same executable file, they are definitely combined in one program. [...]
By contrast, pipes, sockets and command-line arguments are communication mechanisms normally used between two separate programs. So when they are used for communication, the modules normally are separate programs. [...]
