
Gmail compose window is small and composing, and correction takes lots of time. Google has introduced Smart Compose in Gmail, which will help you to autocomplete Gmail text when you write.
During the composing of an email whatever you write in the Gmail compose window, it will predict the text. With the help of the autofill feature, you can write an email effectively.
When you write an email to someone Smart compose allows you to write in a better way by suggesting some useful phrases or words.
Smart compose is a Gmail smart feature for users who use Gmail on their desktop. It will help you to write emails faster and better by suggesting words and phrases.
Gmail Autocomplete Turn ON
To turn On autocomplete open a “Gmail” account on your desktop.
Click on the Gear icon then See All Settings.

In the panel under Smart compose select Writing suggestions on and then save your changes at the bottom.

You are ready to use Gmail autocomplete. When you write the first email to someone it tells you about smart compose and prompts you to press the Tab key whenever you see a prediction.

Gmail autocomplete Turn Off
Open Gmail Account. Click on the Gear icon then click See All Settings.
In the pane under Smart compose, select Writing suggestions off and save your modifications at the bottom. This will turn off autocomplete in Gmail.
Things to be remembered
Suggested text to complete sentences will appear in the background of your email in lighter text. Prediction is available only in English.
The instruction to turn OFF writing suggestions does not turn it off. There is NO WAY to turn it off and I Hate the feature!
If it doesn’t work for you, then you can try to turn OFF Personalization-
Go to Settings > General and scroll down to “Smart Compose personalization.” Select Personalization off.