Hmm... I don't see ads on my Gmail mobile client. I see ads on Facebook mobile client or too small to notice (I do notice the "recommended posts" sometimes). Free iPhone apps come with "ads". I personally don't see how mobile ads are so effective as desktop.
Your mobile gmail client will show ads on the promotions tab. I agree that you might not see those.
People are saying the future of advertising is mobile, not because it presents a good platform for showing ads, but because increasingly that is how people are spending their time, so there is an expectation that advertising dollars will follow.