Hi Alexander,
I moved to new version 26.5 and here my results:
Please see:
Test data(input/output/QGIS)
5400 Gml GeoPackage.zip (94.5 KB)
Thanks,...class (there may be more than one in the GML) foreach (var featureClass...automatically; // you can also force one: gpkgWriter.Crs = CrsHelper.GetCrsById(4326);...