If D-Touch is effectively a port as either a translation in another programming language or a modification of the original such that it can run on Android, then the original Doom license would apply whether it is GPL or anything else (in the sense that this is the original code or clearly a derivative work).
If D-Touch is a complete rewrite just using ideas of Doom, it may still be considered a derivative work. This is for a court to decide, but think about this: say you write a novel from scratch that is a sequel to Harry Potter. Or you write a comic book on the X-men. In both cases, say this is stuff that you wrote entirely yourself. Nevertheless it could still be considered as a derivative work (and therefore eventually infringing on HP or XM copyrights). And the respective author would quite likely be on your number. And then drawing a parallel with D-Touch, copyright may be infringed even if the code is not actually "copied" (whatever the original license was, this is not GPL-specific). This definitely would need the review of a lawyer as the parallel between a book and source code may not apply at all.... I am NOT a lawyer.
That said, I can read this note at the bottom of beloko.com:
All ported games from Beloko Games are based on the GPL License and fully complient with its regulations.
So they claim to be "complient" meaning that source code is likely available. I cannot discuss whether it is indeed "compliant" but I can see source released here https://github.com/emileb/OpenGames and https://github.com/emileb/
And if this is not satisfying to you, you could contact the author to request the corresponding sources.
In more general terms if the GPLed software is left untouched and the additional code is only a 'wrapper' that calls the code. Will GPL extend to the 'wrapper'?
It depends what is the wrapper and how it interacts with the GPL-licensed code. There are several QA on this topic already. The way things interact could create a "combined" or "derivative" work or be just plain side-by-side distribution.
In the specific case of Doom (likely in C/C++) and Android (likely called from Java code by a native layer of sorts such as NaCL or JNI or JNA) my take is that the GPL would extend to such a Java wrapper if this is really how these things work.