Using Outlook for iOS for personal email and calendar management
Hello Cecilia Iris Guido-Spano Amoretti,
I think there may be some confusion between the apps here. You do not need to install the Microsoft 365 app instead.
Please reinstall Microsoft Outlook from the App Store. When you open Outlook, it may automatically detect an existing email account. If it shows the Gmail account you do not want to use, select Skip, then enter only the email address you want to use with Outlook.
If the Gmail sign-in prompt still appears after doing that, tell me at which point it appears and we can narrow down what is triggering it.