This part is critical because if you choose the wrong one then you could boot up to a flurry of force closes. If this changes in the future and you end up getting a custom ROM based on Android 6.1(or whatever the next version is called), then just be sure to download the Gapps package for that version of Android. As of writing this, there are only 6.0 and 6.0.1 custom ROMs on the market so be sure to pick a Gapps package for Android 6.0. Once that is done, you’ll want to download the appropriate Gapps package that matches the same version of Android that your custom ROM is based on. Just like with custom kernels, each custom ROM has a goal in mind and it is best to look through the options and see which ones you agree with the most. I will be compiling a list of all the custom ROMs I have found for the Nexus 6P soon, but for now you can head into the Android Development and Original Android Development forums for a look at what is to offer.
I get all of my custom ROMs from the Nexus 6P XDA forums so I would recommend that you do the same. Naturally you will need to download a custom ROM for the Nexus 6P that you want to install.
INSTALL A CUSTOM ROM IPHONE 7 INSTALL