Is it possible to use Gmail properly via Outlook 2015 for Mac?
Interestingly enough, I was just searching for a good mail app, and found that there is a new Gmail desktop app coming out in the next week. I'm going to try it, because I'm not quite satisfied with the other mail apps, and I like google's formatting. To manage multiple email accounts, you may consider having your other emails forwarded to your gmail account. That's what I've been doing. It's a temp fix for what you're looking for, but may help. I hope so.

Log in to your other email that you'd like to forward and follow similar instructions.