3 Lines top left then select inbox. To change which one you want top right will be a colored letter and select different account. 

TENKAN: Have you added each account separately in Gmail? as you would need to do this to switch between accounts.
Also there is a setting when you open the default account to show just one account or all of them, it's called show All Mail, uncheck that and see what happens.

Can't find that setting any where 

shrub:

 

3 Lines top left then select inbox. To change which one you want top right will be a colored letter and select different account. 

 

That's the problem only showing all accounts combined. No list of other accounts to select from

Ok I have found it. ---- On the top right is a little icon of the account showing. If you tap that you get your account lists and can change them. Oh so very intuitive (not)