Ads start appearing in Gmail's promotions file
If you're upset by this, there really is no reason to be. The ads are tastefully disguised as email messages and besides, you never know when something you see an ad for is something you've been looking to buy for a long time now. And if you don't open the promotions folder, you won't have to look at one paid ad at all.
Just keep in mind that ads are what keeps free content and free apps free! The ad in the screenshot below is for business card printer Vistaprint and while it is in another language, you can clearly see where it is marked as an advertisement so you could skip it quickly if so desired.
source: AndroidWorld (translated) via Phandroid