When attempting to call gennet40 without any arguments you are given the following help documentation:
 

>gennet40.exe
Usage: gennet.exe -l<log filename> -o<output filename> <assemblys to load>
-l Filename used for logging failures
-o Filename for the output file
-d if you do not want to include dependent assemblys
-impdir Logical for generated imports

However this documentation appears to be incorrect (the first thing that should be noticed is it talks about gennet.exe as opposed to gennet40.exe , using -o gives the following:
 

>gennet40 -ointerop.dbl CSharpInterop.dll
Invalid command line option

Reading the documentation (https://www.synergex.com/docs/index.htm#tools/toolsChap4Gennet40utility.htm) indicates that this argument should be called like this
 

>gennet40 -output interop.dbl CSharpInterop.dll

Which works.

The usage documentation should be updated to reflect the correct usage or the tool should be fixed to support -o