By G-mail you mean Google... And no, they don't "read" your email.
Google detects what you have been searching for on the internet. That way they learn your interests and gives you ads that are more suitable for you. You are gonna get ads no matter what (unless you install some kind of adblocker)
Tell me.. would you rather see an ad about cars or an ad about makeup... just asking.
If you are sick of ads, download chrome adblock.