Gmail's smart compose feature has been available on desktops since its announcement at Google's I/O conference in May 2018. However, its availability on mobile devices has been limited to the Google Pixel 3 series.
Now, it appears the tool has gone live for other Android smartphones such as the Pixel 2 and 2 XL, OnePlus 6T, and Samsung Galaxy S9+, among others. It's important to note, though, that the Gmail app must be updated first to the latest version. Once that is done, a pop-up window will appear introducing the smart compose tool when you open the app.
It works in the same way as it does on desktops, providing you some word suggestions as you start composing your message. The artificial intelligence-powered feature uses context-aware methods to help you finish a sentence, thus saving you time typing several words. However, you will need to swipe on the suggestions in order to allow them into your draft, which is different from how it works on PCs where you'll need to press the right arrow key.
At first, the feature is turned on by default, but there's an option to disable it if you want to compose messages your own way.
Source: Android Police