When I access my Gmail through the Google Gmail icon on my phone home screen, at the top of my mail categories like promotions, forums, social etc there are ads. I slightly exaggerated though.....I get dating website ads among others that are very suggestive, but not pornographic. It's not a virus or malware....it was there from day one, and I don't surf on my phone and have only installed one app..... Fitbit.
It's built into the Google mail app on the phone. They don't show up in my primary category, just the sub categories.
I get ads in my Outlook accounts as well, but they're based more on Microsoft products etc, not like the
Gmail ads I find offensive.