1-2 of about 2 matches for site:docs.gimp.org demonstrate transparency
https://docs.gimp.org/2.8/en/gimp-using-web-transparency.html
2.3. Saving Images with Transparency 2.3. Saving Images with Transparency 2. Preparing your Images for the Web
https://docs.gimp.org/2.8/en/gimp-using-web-transparency.html
2.3. Saving Images with Transparency 2.3. Saving Images with Transparency 2. Preparing your Images for the Web