Since FME writes a Geotransform table into the metadata, you can just take that for your mosaik file:
Transform your file to VRT
gdal_translate -of VRT mosaik.tif mosaik.vrt
Open mosaik.vrt in a text editor and insert a Geotransform line:
<VRTDataset rasterXSize="15000" rasterYSize="25000">
<GeoTransform> 2575426.714515543, 0.011400621271025, 0.004378508269806, 5729751.965613389, 0.004376888527729, -0.011404849098262</GeoTransform>
<VRTRasterBand dataType="Byte" band="1">
<ColorInterp>Red</ColorInterp>
<SimpleSource>
<SourceFilename relativeToVRT="1">mosaik.tif</SourceFilename>
<SourceBand>1</SourceBand>
<SourceProperties RasterXSize="15000" RasterYSize="25000" DataType="Byte" BlockXSize="15000" BlockYSize="64" />
<SrcRect xOff="0" yOff="0" xSize="15000" ySize="25000" />
<DstRect xOff="0" yOff="0" xSize="15000" ySize="25000" />
</SimpleSource>
</VRTRasterBand>
</VRTDataset>
Alternatively, you can create a custom omerc projection around the image center, as described in How to create proj4 definition of local coordinatesystem
+proj=omerc +lat_0=51.6959777875 +lonc=7.0923165808 +alpha=-20.145 +gamma=0 +k=1 +x_0=50692.579 +y_0=81723.458 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs