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There's no surefire way to get this done. A contact can open his email in Gmail once and in Outlook the next time.
In order to get close to what you're trying to do, you can create Smart Lists based on the device used to open one, some or all of your emails. The more emails you choose, the bigger the chance someone will be on 2 multiple lists at the same time.
Create a Smart List, select the device / platform / browser combination you're looking for.
In the Flow steps for your 'Send Email' click the link 'Add choice' and select 'Member of Smart List'. If they're in the Gmail smart list, send them the Gmail email. If they're not, send them the outlook email.
Hope this helps.