Unfortunately that's pretty much the deal with a free gmail account. If you don't want them to do it, pay for a business account (maybe?) or pay for an account

Unfortunately that's pretty much the deal with a free gmail account. If you don't want them to do it, pay for a business account (maybe?) or pay for an account somewhere else. Mining your email for keywords and selling you things has been part of their business for a long time. Knowing what you've actually bought is just part of that.