And if you want some customisation, e.g. some repeating string over and over, you can use something like this:
yes "b0M" | tr -d '\n' | head -c 10G | gzip -c > 10GB.gz
yes repeats the given string (followed by a line feed) indefinitely - originally meant to type "yes" + ENTER into prompts. tr then removes the line breaks again and head makes sure to only take 10GB and not have it run indefinitely.
If you want to be really fancy, you can even add some HTML header and footer to some files like header and footer and then run it like this:
yes "b0M" | tr -d '\n' | head -c 10G | cat header - footer | gzip -c > 10GB.gz
There were ads! But these were simple banner graphics of 468x60 pixels. In the worst case it was an animated GIF. But hosted on the same server as the page and without any tracking shenanigans.