The Microsoft Store's app offerings may be sparse already, but arguably the most ominous omission is the lack of any Google-related. The Palo Alto giant has made it a point to offer, at best, minimal support for Microsoft's app platforms, and will likely not bring a Gmail client to the Microsoft Store for a very long time, if ever.
If you're a Gmail user who's not entirely satisfied with the native Mail client included with Windows 10, functional though it may be, and want the full suite of Gmail-specific features and functionality, WunderMail for Gmail may be the app for you.
Built on top of the Gmail API, the app supports Gmail-specific features that Mail for Windows 10 and any other IMAP-based email clients you might find on the Microsoft Store don't. This means your emails are sorted just like they are on the web client for Gmail, with support for automatic app sorting into different categories like 'social' and 'promotions'.
WunderMail may be made for Google mail, but it's designed for Windows 10 through and through. It's a native UWP app and embraces Microsoft's Fluent Design Language heartily. Sporting a gorgeous UI with acrylic and red accents, it even looks better than Microsoft's own client. There's also Action Centre integration so you don't miss any important emails, and the developer is working on adding dark mode to the app in the future.
If all of this has you interested, you can download the app from the Microsoft Store here. It is free to use, but does contain two ad banners placed among your emails. You can go ad-free by upgrading to the app's premium service, which costs (the equivalent of) £8.29 per year.