The Android app Octave in Google play is built and maintained by Corbin Champion and not part of the GNU Octave project.
The source and some build instructions for the installer can be found here
Because the app urges you to donate a fee some users asked on the mailing list if this is a GPL violation. One example Octave for Android Requires Payment
There is nothing wrong with distributing binaries to only those people who pay for them. The GPLv3 requires some form of source distribution if you distribute binaries, but it doesn't require that you distribute binaries free of charge. If you're offended by Corbin charging money for his work in getting Octave working on Android, the source above makes it possible for you to duplicate his work and avoid paying him by undertaking the chore of building Octave for Android yourself. Any person with enough skill could take the Octave sources, Corbin's GitHub repository, and the Android SDK to reproduce his work without the need to pay for anything. Here is the donation dialog