Issue #2 was closed, but
when testing reprojecting Gauss-Krueger Zone 3 to WebMercator a wrong offset persists.

Geotrellis seems to use some wrong parameters in proj4J (Java https://trac.osgeo.org/proj4j/) library. This issue has been seen before with GDAL library, where proj.4 (C/C++ https://proj4.org/) is used. In GDAL this seems to have been fixed (probably csv table with parameters was updated).