Double Click on the .OVA file. It will open automatically a VirtualBox interface.
Click on Import.
Modify the following settings parameters:
On General, modify the Version to Ubuntu (64-bit), as shown below.
On Storage - Attributes, modify the Type to AHCI, as shown below.
Once you have done all these steps, you can start working on your ThinMan Gateway Virtual Appliance G1000V. It might be necessary to change the resolution on the Video Settings as shown below.