So for anyone interested in "Upgrading" your 20+ month old Studio Energy to some more current and appealing ROMs. It is true that you can Flash Roms for Gionee M3, if you do it right. I am currently running a Marshmallow 6.0.1 version of AOSP called "Resurrection Remix" using Kernel 3.4.67. I got this, and a handful of other ROMs form
here.
Follow these steps precisely to achieve a successful flash to these custom ROMs. For American D810u devices, if you flash these Roms, you will loose 4G (which is not LTE) capabilities because its missing the 1700 baseband. However there is a patch that was created some time back that if flashed correctly, will enable 3G (H+) baseband.
Note: depending on which ROM you are flashing, you may need to be running either a KitKat kernel or a Lollipop kernel prior to flashing. I believe these Roms specify which one you NEED to have before attempting this.
Step 1: Make sure the Zip files for the Rom, Gapps, and the patch file located on this thread
Post# 91 are all located on your devices internal memory (remove external SD card before flashing)
Step 2: Using TWRP 2.8.4.0, make sure you are mounted to the devices internal SD card, do a factory reset ( wipe data, cache, dalvik).
Step 3: Flash the new ROM, Gapps, and the patch file, then wipe the cache and dalvik, and reboot system.
Give the device 5 minutes or so to boot the first time. You will find that your device is running a Gionee KitKat, Lollipop, or Marshmallow Rom that also has our american baseband so 3G will work. Have fun with the new ROMs!!