timeSinceGPSEpoch is completly wrong when i look into live lorawan frames of the gateway.
The problem is that my end device takes the wrong time for time synchronization (DevTimeRequest).
The GPS coordinates on my gateway are correct.
Can anyone tell me why the wrong time is shown there?

It does seem like it should be closer to 1.39 billion seconds.

Yes sure, but where is it calculated?

It comes from your gateway. Could you share the gateway model that you have and which packet-forwarder you are using?

its a Tektelic Kona Enterprise with a semtech packet forwarder.

To rule out the issue is with ChirpStack or with the gateway, what you could do is capture the raw UDP data, see one of these commands: