The Android Police site reported version 4.6 of the Gmail app for Android, on handsets running Android 4.0 upwards, contains multiple mentions of advertisements within the application code, including references to saving ads as messages and dismissing them.
Google, increasingly seeking ways to create income from its mobile properties, added an ad feature to its Google Maps app on both Android and Apple's iOS in August, with ads showing up with searches for places, services and landmarks.
"The implementation of the adverts within the mobile Gmail apps will be crucial, due to the impact they could have on the limited real estate available on mobile devices," Ian Maude of research firm Enders Analysis told Britain's The Guardian.
"It will be a balancing act between designing a system that is prominent enough to attract attention while not annoying users; it has to be intrusive enough to garner a response," Maude said.
Google could have a range of options in how it presents ads in Gmail without risking users' ire, he said.
"It would have to be fairly bad to make people switch email providers as there's quite a lot of inertia -- including the hassle of notifying all your contacts of a new [email] address -- preventing easy switching," he said.
Asked about possible ads in the Gmail app, a Google spokesperson told The Guardian, "We're always experimenting with new features, but have nothing new to share at this time."