Hello Dominik,

the error you get is due to the fact that the
gem you are trying to install is building the software
from source code.
This is usually done in three steps - 

configure/make/make install ... 
but the Windows version of nmake is not installed on your computer.
You can install a compiler, such as MinGW :

http://www.mingw.org/

on your system to do the compilation.

This can be useful in many cases anyway...
I haven't worked with specific backpropagation software, but
if you want to use Ruby for scientific work, some general
numerics software such as Ruby-GSL (http://rb-gsl.rubyforge.org/)
which is included in the one-click installer for Windows

http://rubyforge.org/frs/?group_id=167

may be helpful.

Best regards,

Axel 


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