So you want your torify your Linux Mint VM’s traffic through the Gateway, correct? Try the following:
In VirtualBox, select Linux Mint VM -> settings -> Network -> Chose Virtual Network/Whonix
Save and start the VM. In the VM, edit the /etc/network/interfaces file with a text editor and admin rights, or just type:
sudo nano /etc/network/interfaces
and type the following:
iface lo inet loopback
iface enp0s3 inet static
sudo nano /etc/resolv.conf
and replace everything with the following line:
Reboot the machine and test it.
This way, you have configured the VM to use the Whonix-Gateway as its gateway (at local address 10.152.152.10) and it will assign the static local IP 10.152.152.12 to your VM. Use another last number if you wish to run your VM alongside Whonix-Workstation which also uses 10.152.152.12 (chose for instance 10.152.152.13).
I don’t know the purpose of this for you, but please note that Whonix-Workstation provides much better security than a simple torified VM.