In case you have used spaces before and after the = sign, try without them. You might also want to try with gl instead of ngl.
The one-line command for it would be:
echo export GSK_RENDERER=gl | sudo tee /etc/profile.d/gtk_renderer.sh > /dev/null
Remember the name of the created file (and/or use a more suitable filename), so that you can revert in the future.