Setting Up Your Custom MailChimp Template
Using a custom template will give you a much better chance of hitting your subscribers' inbox – and avoiding Gmail's Promotions tab – compared to using MailChimp's built-in templates.
But, hitting the inbox is a combination of a number of factors...time and domain trust being the most important ones.
So, even though you're using a custom template, it's not a magic bullet to keep you out of Promotions. But with time as you email more, get more engagement and replies, send quality content that doesn't get spam complaints – all things that improve your domain trust – you should see great improvement in your inboxing rates.
In my own case, here is what happened when I authenticated my domain with MailChimp and switched over to a custom template like the one in this video:
BEFORE:
AFTER: