Google introduced Smart Compose for Gmail on the web back in 2018 before rolling it out for Gmail on Android in 2019. The company is now testing bringing Smart Compose to other messaging apps on Android as well via Gboard. The latest beta of Gboard has enabled the Smart Compose functionality for many people in messaging apps like Messages, WhatsApp, and Telegram.
WIth Smart Compose, Gboard will automatically give you a suggestion to help you complete the sentence that you are typing. This is different from the suggestions made by the keyboard as Smart Compose will suggest complete sentences for you while the former only has the next word suggestions. If you like the suggestion that's made, you can swipe right on the text that appears to accept it. Users will also get a quick explainer on Smart Compose the first time around a suggestion appears.
As of now, Google is beta testing Smart Compose with selected Gboard users running v184.108.40.2067844448 and it is unclear when the company will start a wider rollout and what apps will be supported by it.