Gmail doesn't need to sell data to anyone, they use it for rest of the google suites like google.com, adsense, youtube, doubleclick, and all the other properties they own.
In fact it would be a stupid idea for them to sell any of that data directly to a 3rd party. Instead they package them in user friendly (marketer/advertiser friendly) ways to capitalize. Some of these are shady and I'm not a fan but overall I think this approach is fine.
The problem happens when you sell user data to a 3rd party.
Here's an example: Let's say you start an email newsletter about travel. You get millions of subscribers. Then you start putting ads on your email. Maybe sometimes even send sponsored messages. This is kind of annoying but not "unethical".
On the other hand, the same company could take all the email list and sell it to bunch of travel agencies. Then all the million users who subscribed suddenly start receiving spam emails from these travel agencies. This is unethical because they literally "sold" your email address.
Of course this is more of an extreme example, but the pattern is the same.
In fact it would be a stupid idea for them to sell any of that data directly to a 3rd party. Instead they package them in user friendly (marketer/advertiser friendly) ways to capitalize. Some of these are shady and I'm not a fan but overall I think this approach is fine.
The problem happens when you sell user data to a 3rd party.
Here's an example: Let's say you start an email newsletter about travel. You get millions of subscribers. Then you start putting ads on your email. Maybe sometimes even send sponsored messages. This is kind of annoying but not "unethical".
On the other hand, the same company could take all the email list and sell it to bunch of travel agencies. Then all the million users who subscribed suddenly start receiving spam emails from these travel agencies. This is unethical because they literally "sold" your email address.
Of course this is more of an extreme example, but the pattern is the same.