Maybe you're stuck on what to say next, or perhaps you're just looking to save some time. Either way, Google can help you out.
Google's Smart Compose feature completes your sentences as you type. If you write something such as: "What should we get for dinner next week?" and then start your next sentence with "Let," Gmail will likely suggest the words "me know" to complete the sentence.
To enable this feature on the desktop, click the Settings cog and scroll down to "Smart compose" under the "General" tab. On your Android phone, tap the symbol that looks like three horizontal lines stacked on top of one another in the Gmail app, choose Settings, select your account, and scroll down to "Smart Compose."
This feature isn't available on the iPhone yet but will be coming at a later date.