jugar

I can also confirm the ROG phone 5s works well on T-Mobile. 4g lte, and 5g work with strong reception. Voice over LTE works. To get visual voicemail I had to download T-Mobile app, it was not available in default dialer.

Only thing that is not working is wifi calling.

Asus' ROG 5's works very well on T-Mobile.  I wish the Xiaomi Black Shark's were more T-Mobile friendly though for 5G.  They work, just not as well as the Asus model due to band compatibility.  Ultimately, it depends on what you are looking for in a gaming phone.  Performance in games, you might want to consider Xiaomi because their devices run a lot cooler than the competition while maintaining their peak performance.  If you want a better 5G experience, I would go with one the ROG 5's.  If you want to use the device very long term, the ROG 5s with 18GB of RAM stands out. 

Anything else, I wouldn't really consider outside of the non-gaming focused devices sold by T-Mobile.  Most flagship devices will perform well in games.  Just look for devices without heavily skinned and bloated UI's.  

Most good gaming phones aren't sold BY Tmobile like the Asus phone line. @syaoran may have a better idea of what 3rd party phones play well on Tmobile.