I do this in the gmail mobile app all the time thanks to their ads injecting themselves in between the emails several seconds late. They must love me, their most engaged ad consumer.
<sarc/> A website can track where your mouse pointer is, but surely they wouldn't do this on purpose just to get ad revenue.
Handy tip: If you disable all the tab category things in settings and go down to a single category (Primary), it will stop showing you ads entirely.