Sorry, found another issue.
In the changelog it is said that now pixelRegion and geometryRegion can be used. (SNAP-3715 Allow subset region (geometry or pixel) from GPT)
But how? The help page has not been updated and when invoking gpt -h from the command line the same help is printed. The CommandLineUsage.txt has not been updated.
I guess it is intended like
-PpixelRegion=<value> and
-PgeometryRegion=<value>
Can’t this interfere with parameters of some operators?