To change the language of your Gmail account on an iPhone, you will need to change the language of your device. Here's how to do it:
Open the "Settings" app on your iPhone.
Scroll down and tap "General".
Scroll down and tap "Language & Region".
Tap "iPhone Language".
Select the language you want to use for your iPhone and Gmail account.
You may be prompted to confirm your choice and to wait while your iPhone downloads the necessary language files.
Once your iPhone restarts, your Gmail account should be displayed in the language you selected.
Note that changing the language of your iPhone will change the language of all apps on your device, not just Gmail. If you only want to change the language of Gmail, you may be able to do so by changing the language setting within the Gmail app itself. However, the steps for doing so may vary depending on the version of the app you are using.