Simplify Gmail is a Chrome or Firefox browser extension that brings the simplicity of Google Inbox to Gmail. Michael Leggett is the creator of the extension who was previously Gmail’s lead designer from 2008 to 2012 and co-founded Google Inbox. So it’s made by someone with deep knowledge of the product.
If you miss the simplicity of Google Inbox then Simplify Gmail might be the closest you’ll be able to get. You can find more information on its website which includes links to download the extension in the different browsers.