Gmail for Exchange or Microsoft Exchange Active Sync
There are options here.  You can remove accounts in Settings - Cloud and Accounts - Accounts (based on S7 Edge running Android 7.0):


Currently the way you are doing it
Remove all accounts, log into Gmail with your personal Gmail account, open the Mail app and use "Exchange" to connect to your Office 365 account
Use Gmail to access your Gmail account, then also add your Office 365 account to view in Gmail app (Gmail app supports Exchange accounts)
Use only Mail app to access Gmail and Office 365 accounts
Use Gmail and third-party app for Office 365 access (Outlook, Nine for Android, TouchDown, many others)


There's a lot of options here.  It is up to the user how they best organize and use their apps.