Since Google usually ship key features of Gboard in iOS before delivering it's to Android users, users of the green robot that are amazed by this feature will need to wait a bit longer to get this seamless translate feature.
Google updates Gboard for iOS, adds translation support for 103 languages
"As you type, you'll find the translation in the suggestion strip or by pressing the Google button". Tap that option to pull up the direct text translation feature. This new update brings Google Translate support and with this support, you can now use Gboard to translate to and from all the languages supported by Google Translate.
Although 103 translation languages sounds like more than enough, Gboard actually supports 685 languages for you to choose from for typing with, so the translation system remains relatively limited. To date, Gboard has language support for nearly 700 languages. Just tap the Translate icon and you can then type or paste in text. The Translate feature remembers the previous output language, speeding up future translations. Gboard from iOS is available from the App Store.