Installing the Open GApps from the widget is currently broken with Genymotion Desktop 3.1.2 and below. A fix will be released in Genymotion Desktop 3.2.0. It is already available in our 3.2.0 beta version at https://www.genymotion.com/download-beta/ .
In the meantime, it is still possible to install the OpenGApps manually. To do so, please follow the instructions from How to install OpenGApps manually?.