Hey Martin,
I don't think anybody used Google Cloud Pub/Sub with Flink yet.
There are no tutorials for implementing streaming sources and sinks, but Flink has a few connectors that you can use as a reference.
For the sources, you basically have to extend RichSourceFunction (or RichParallelSourceFunction).
As long as the data rate is low, you can start with a non-parallel source implementation. This makes the implementation a bit easier in the beginning.
Let us know if you need more help.
Robert