Release updated source code
It seems like the source code published here is outdated from the version on Play store. I could find a reasoning the you don't want others to copy the project so you delay the release of source code. This goes against the idea of open source software and the licence GPU GPL that you use.
- You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
a) Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange
Release of the source codes would allow contributors to improve Plus messenger as well as asure users that the software they use can be inspected and is safe.