If you have more than one GQueues account, you can still access all of them from the mobile app.

To add another account choose Accounts... from the sidebar menu.

Then tap the Add Account button.

Login with the credentials for the other account.

To switch between accounts choose Accounts... from the sidebar menu and then tap the account you want from the list.

Choose Accounts... from the sidebar menu.

Tap the gear icon for the specific account.

Choose Remove Account. This will entirely remove the account from your device. The web version of your account will remain intact, but its data won't be accessible from your mobile app anymore.