Google is awesome...in a scary sort of way

So I had a trip to Pittsburgh coming up where I was staying in a hotel for a night. Since I have best access to my Gmail from the road, I forwarded my hotel confirmation to that email account a few days before my trip. Within hours, my Google+ enabled smartphone started giving me weather updates for Pittsburgh, in addition to my home location. Hmm, I thought, how did my phone know I was going there?

I speculated the source was the info I sent to Gmail, so I tried an experiment and sent my flight confirmation to Gmail. Sure enough flight updates starting popping up on my phone and watch (yes, a Google/Android wear watch, I'm firmly entrenched).

These updates and the fact that they showed up nearly automatically are awesome and I've taken to using this method whenever I travel, but what is behind all these updates is that Google is reading my email. I'm not sure how I feel about this, since I don't use my Gmail much. But a lot of folks out there do use it as their primary email. So how how much IS Google reading emails? How much info is retained/stored/distributed/sold?

For now, it's a price I'm willing to pay for the gained awesomeness, but perhaps a day may come where I value my privacy over the added functionality. What about you, what's in your Gmail?