It took me some time to find out how to exactly forward the GUI applications from multipass machines to host, but it turned out to be simple...
First, we need to 'ssh' to our guest machine.
Multipass offers the 'shell' command, which is also a wrapper around ssh. Since we are required to turn X forwarding on, we will use the original 'ssh' command instead.
Since, ssh requires public key, we first need to get those keys and save it to our home repository. The ssh keys are saved in '/var/snap/multipass/common/data/multipassd/ssh-keys'. It turned out that multipass uses same key for all the container initiated, so, we just need to perform this once.
sudo cp /var/snap/multipass/common/data/multipassd/ssh-keys/id_rsa ~/.ssh/multipassKey/id_rsa
Add the key to your grp( replace user with your user name)
sudo chown user -R .
Start the container with the 'name'
(replace it with the actual name)
multipass start name
Find the ip of the running container
Now, ssh into the machine with ip 188.8.131.52 ( replace with ip found above)
ssh -X -i ~/.ssh/multipassKey/id_rsa firstname.lastname@example.org
Now, all the GUI applications will be forwarded over ssh to the host machine