You won't be able to access the feature that you're talking about no matter how many accounts you have set up.
I also have multiple Gmail accounts and they are all set up on my iPhone & iPad. You will not be able to duplicate the action that is allowed on a computer, the choice from a drop-down menu of which account to reply from. You send and receive from the account that you happen to be logged into on the mobile that you're using at the time. If you want to send from another account you have to login to that account and compose the email there. On my computer I have all of my Gmail accounts set up to forward all emails to my main account so that I don't have to check them all every day but don't miss any messages. When I reply to an email on Apple mobiles the reply goes out from my main Gmail account regardless of which of my accounts the original email went to unless I log out of the main account & login to the account which originally received the email. It's a PITA to bother with.
I tried to work around this issue earlier in the week and it's just not possible, AFAIK. I even logged into Gmail using Safari & Atomic Browser; Gmail just doesn't behave the same way as on a computer.
If anyone has figured this out, please share. I gave up until there's a real full-bodied Gmail app that is stable, at which point I'll try again.
UPDATE: It works! Help from Post #6 was the candle in the darkness. Thanks!