An Android app has a UI written in Java, not on top of a C/C++ based GUI toolkit (GNOME, Qt, KDE, wxWidgets, …). Gnulib cannot help you porting this part of an application to Android.
But when an application has a large part written in C, and you want to reuse this part in the Android app, Gnulib is useful. Recently, Android has been added to the list of portability targets of Gnulib. This means that most of the POSIX functionality that you might use in your application will compile and work fine also on Android.
In particular, the work-in-progress port of GNU Emacs to Android uses Gnulib to its best.