many thanks for your answer (and sorry for the late answer).
It now works fine and the plugin has been successfully downloaded (though I still get the “eval is not recognized as an internal or external command” error message).
Preparing to install Google Maps [v1.0.4]
|- Downloading package… 100%
|- Checking destination… ok
|- Installing package… ok
'- Success!
I followed your suggestion, i.e. I installed Git for Windows. After the install, I could run the gpm install and it downloaded the plugin (and its dependencies).