Literally since the beginning AFAIK. Although they allegedly stopped scanning e-mails for targeted ad data years back: https://www.pcmag.com/news/google-to-stop-scanning-gmail-messages-to-serve-up-ads

I don’t recall seeing ads in the phone app, however, just the webapp, so perhaps that is new? Which makes some kind of dark sense given less people use computers to do things anymore, and every tech company is trying to pull off increasingly maximum grift over the last few years.