I believe this has actually been in the works for many, many years, but now they are publicly promoting this capability. You can read How Gmail Ads work
Personally, I consider this unethical because emails between you and your family, friends, clients and others should be PRIVATE – not data-mined so ads can be targeted at you.
I have a better way. I don’t use gmail.
Oh, I HAVE gmail accounts. You can’t work online and avoid them easily. (I even told Buzz NO but it appeared anyway.) I just don’t use them. But since many of the people I email to do the emails I send them are data mined.