I've been seeing this error for a while now when trying to drag-n-drop a gapps file to a running emulator instance.
The problem lies between your version of android tools/sdk and the one deployed with Genymotion.
If you switch back to the default bundled version then that fixes the problem with not being able to flash files via drag-n-drop.
You will need to switch back to your version of android tools in order to use adb to install and debug applications.
You do this via the "ADB" section of the Genymotion settings.
I'm using genymotion 2.6.0 and VirtualBox 5.0.18.
