I am trying to email all customers at same time however, when it brings me to Outlook email and I need to send from Gmail account.

Hi there, 

It's great to have you reach out to us in the Community. I'll be happy to share more info and help steer you in the right direction. 

To get started, I'd like to make sure I'm on the same page as you. Could you walk me through each individual step you're following so I can replicate this on my end? Feel free to respond to me in the comments below. Also, you're welcome to send screenshots for visual reference if you'd like. 

I look forward to hearing back from you. 

Sure. I go onto my Quickbooks online, from the left panel I click Customers. From there I select all the customers. Then in Batch Actions, I select Email. I need to email from my Gmail account however, Outlook opens

I see what you're saying. Could I have you log into your account using a private/incognito browser window and let me know if you see the option for selecting Gmail when you follow the steps you sent above?