Signal doesn’t work without Google Services (they even use Google Auth to login…), but it works with microG (Implementation Status · microg/GmsCore Wiki · GitHub).
Also, don’t be fooled: Signal doesn’t anonymize any metadata and it’s centralized. Snowden used it because he ran it over Tor. So using Signal means at least Google (by GCM) and Open Whispers Systems (by their servers) have access to the metadata of your comms.
“It’s very good, I know the security model,” Snowden said about Signal. “They don’t protect you from metadata association, but they do strongly protect your content from precisely this type of in-transit interception,” he said.
I’m still waiting for Briar for a full encrypted IM app.