Signal is open source, so you can just fork it to remove the dependency on GCM.<p>In fact, someone already has, so you can just use it:
<a href="https:&#x2F;&#x2F;fdroid.eutopia.cz&#x2F;" rel="nofollow">https:&#x2F;&#x2F;fdroid.eutopia.cz&#x2F;</a>