There is no system.prop in the Genymotion OS build : the file is merged with build.prop which is partially read-only.
To modify these product properties, you need to use gmtool with the admin edit and --sysprop options:
gmtool admin edit "Virtual_device_name" --sysprop=<PROPERTY:VALUE>
For example, to change the model property value to "new_phone" for a virtual device called Samsung Galaxy S9 :
gmtool admin edit "Samsung Galaxy S9" --sysprop=MODEL:new_phone
Available properties are: MODEL, PRODUCT, MANUFACTURER, BOARD, BRAND, DEVICE, DISPLAY, SERIAL, TYPE, FINGERPRINT, TAGS. You can set multiple system properties.
For more information, please refer to GMTool user guide.
This feature is not available with the free for personal use edition: a license is required.