Motorola officially supports bootloader unlock for the Moto G4/G4 Plus (Moto G 2016), so you can install TWRP recovery and get root privilege on the device. Custom recovery and root access lets tech-savvy users customize their device, install custom ROMs like CyanogenMod, backup stock firmware and let do various other things.
Motorola will not update the first generation Moto G 2013 to Android 6.0 Marshmallow, but that does not stop developers put their hands on the device. An XDA developer lost101 has ported official stock Android 6.0 Marshmallow ROM for 1st Gen Moto G 2013 (falcon), which you can flash via TWRP custom recovery.
Moto G 4G (1st Gen) will not get official Android 6.0 Marshmallow update, but if you are wishing to get the latest firmware for your device, the CyanogenMod 13 custom ROM will bring it. Cyanogen team has released the CM13 Nightly build for Moto G XT1039, XT1040, XT1045 (code-named Peregrine) with a number of useful additions and features, which will enhance your Android experience.
Android 6.0 Marshmallow OTA update for Moto G XT1068 (2nd Gen) has started rolling out in India. As the OTA is rolling out in phases, it will take sometimes to appear the notification on your Moto G 2014. However, if you have not received the OTA notification message for Android 6.0 Marshmallow update for some reasons, you can manually install the OTA update on your device.
Motorola is known for its timely update their smartphone to latest Android versions. They have just started rolling out official Android 6.0 Marshmallow update for the two-year-old Moto G 2014 (2nd Gen) in India. Owners of Moto G 2014 (XT1068) will receive the Android 6.0 OTA (over the air) update notification shortly on their device.
Moto G 2013 (1st Gen) users can now enjoy Android 6.0 Marshmallow with official CyanogenMod 13 ROM. Cyanogen team has released a Nightly build of CM13 for Moto G 2013 (code-named falcon) with a number of useful additions to Android 6.0 Marshmallow, like the theme engine to enhance your Android experience.