A good place to start reading and understanding what is needed for a GUI

https://help.ubuntu.com/community/Installation/LowMemorySystems

My configuration is below for a minimum system that I consider to be a useable starting point.

sudo apt install xfce4 xfce4-goodies
sudo apt install tightvncserver
tightvncserver :1
sudo apt-get install gnome-icon-theme-full tango-icon-theme
sudo apt install gksu
sudo apt install synaptic
sudo apt install chromium-browser
sudo apt install gedit

open gedit and edit the file /usr/share/applications
change Exec= to “Exec=gksudo synaptic”

Total drive size is 2.144gb when run on the AWS Ubuntu image

Create a partition for the swap so applications have access to more RAM

sudo dd if=/dev/zero of=/mnt/swap.0 bs=1024 count=1048576
sudo mkswap /mnt/swap.0

Add to fstab to automatically mount the partition

sudo su 
echo "/mnt/swap.0 swap swap defaults 0 0" >> /etc/fstab
swapon /mnt/swap.0

Check your work

sudo swapon -s

https://docs.bitnami.com/installer/faq/linux-faq/#how-to-download-and-install-a-bitnami-stack