There are several web resources for the Oder-Neisse-Cycleway. (Oder-Neiße-Radweg in german) Here’s the variant from Zittau to Wolgast I have chosen as a GPX track.

The file is purged, so it just contains the route. It does not contain deviations or markers to viewpoints, museums etc. Its length is about 547 kilometers, and it contains 6014 trackpoints, so it should fit into the memory of current Garmin devices easily.