Posting this in the vain hope that search engines pick it up...on both a former Moto X (2014) and OnePlus One using the new Gmail app for Android (v 5.0.1; new design language) is crushing battery life. Turned my OnePlus One, which could previously go 24 without having to touch a charger, into an ordinary phone that was dead by 6:00 PM. Checking battery usage in Android showed that the Gmail app accounted for 40% of my total battery drain...not good!
I assume the culprit is how Gmail uses IMAP or whatever to check Hotmail. I had it set to synch/fetch every 5 minutes and it simply destroyed battery life.
Removing my Hotmail account from the Gmail app, and using Microsoft's Outlook.com app for Android, immediately rectified everything and battery life instantaneously went back to awesome.
I've seen similar posts noting issues with using the Gmail app to check Yahoo! mail as well. Sad news since ostensibly having the ability to use Gmail to do all email on Android was nice :(