As Run gym-gazebo on Google Colaboratory says, you need to run a framebuffer server (that will emulate a graphical screen) and create a DISPLAY
envvar pointing to it.
!apt-get install -y xvfb # Install X Virtual Frame Buffer
import os
os.system('Xvfb :1 -screen 0 1600x1200x16 &') # create virtual display with size 1600x1200 and 16 bit color. Color can be changed to 24 or 8
os.environ['DISPLAY']=':1.0' # tell X clients to use our virtual DISPLAY :1.0
However, if you want to interact with the GUI, that’s going to be hard, because Colab doesn’t support interactive screens out of the box.