Like I said, I'm fairly sure that breadwallet isn't against Google Keyboard as such, but it is simply because on my ROM (which isn't a nexus factory rom like I'm assuming you're running on your Nexus6P), the "system default" keyboard is AOSP... NOT Google.

I had to download Google Keyboard from the play store and install it. So Breadwallet deems this to be a "Bad Thig™" and refuses to work with it... it wouldn't matter if I was using Flexy or Swype or anything other addon keyboard.

Android already gives you warnings that addon keyboards can steal your data etc... and as long as a person is sensible about what they're doing, and trust the company that coded the keyboard, I don't see why Breadwallet should have an issue with it.