The genetic algorithm (ga) package available on website version 0.10.0 does not work anymore on octave 5.1. please update the package as soon as possible.
Using Linux( openSUSE 15.0) I can still install the ga package with many warnings, but no errors. Admittedly, the code seems unmaintained in the past years (2012 was the last release). What is your operating system, how did you install Octave and the ga packge? Please, can you be more precise about what of the ga package is not working?