Here is how to use Gmail’s new Smart Compose
Gmail’s brand-new Smart Compose feature was revealed by Google at its annual developer meet this week, and it is made available for tryouts as an “experimental feature” for the desktop.

Smart Compose figures what the user might right next this is a way of smart prediction as you type in the body of an email.
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Within this panel, click the box next to “enable experimental access,” and then save your changes at the bottom.
Once turned on, you should see a prompt the next time you begin a new email that tells you about Smart Compose and prompts you to press the tab key whenever you look at a prediction you’d like to accept.

Source: The Verge
As you write, suggested text to complete sentences will appear in the background of your email in a lighter text.
The actual experience of using Smart Compose in the few emails I tried was not very useful, and it often only prompted filling in a single word at the end of a sentence, like in instances of “if possible” or “is this okay.”
It’s very likely that the more you use Smart Compose, the better it will get at recognizing how you write and understanding context, but I found it to be less intuitive than that Taco Tuesday demo that was shown onstage.
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Smart Compose is presently available only in English, and Google gives fair warning that “Smart Compose is not designed to deliver answers and may not always predict accurately correct word.”