Comment by Spittie

I didn't see a link of this somewhere, so I'll just drop it: https://github.com/JavaJens/TextSecure

This is a fork of TextSecure/Signal which drops the dependency on GCM and uses WebSockets instead (it seems quite up-to-date with origin as well).

For anyone wanting a pre-build copy, there is a non-official F-Droid repository hosting both the "main" Signal and the WebSocket fork here: https://fdroid.eutopia.cz/