Towing my travel trailer back from the campground on Memorial Day with my 2013 GL450 with 120k miles, I was going through a cell phone dead zone and the large red battery warning light came on the dash. I did not experience any other issues before I pulled over. I checked the voltage, and it was only 11.9V with the engine running, and that was only 1 minute from when the warning light came on, so it went quick. With that low of a battery voltage, I couldn't risk driving, as my alternator wasn't working and my battery was almost done too. I couldn't get a clean cell phone call in to AAA, but we were lucky that my wife's cousin was also coming home on the same highway, saw us, and pulled off to help. He drove me a few miles to get a good cell signal, we called AAA and a couple hours later got a tow. I did some quick internet research while waiting and found that the voltage regulator is usually the problem, not the whole alternator, and since I had replaced the same type of voltage regulator on my son's BMW I thought I could do it for the Mercedes as well. So I had the GL towed to my house, I ordered the part on the way home with rush shipping, and replaced it in my garage over the last two nights after work. I followed this guy's instructions except I made it very difficult for myself by screwing up a few things. Also, I'm slow in general. It took me 7 total hours from start to fully cleaned up, including time crawling around looking for dropped sockets, but it was worth it because it was cheap and it runs fine now. The problem is that I'm not very coordinated, my garage is tight, and it was very very tricky to get the alternator back onto the bracket, from the bottom of the engine in between all the tubing. You can't see the upper bracket, and there's only room for one human arm to reach in there at a time, the access to the lower bolt is blocked by a coolant hose (which you have to push out of the way), and the plastic cover on the back of the alternator is old brittle plastic and shatters when you bump it against things. One tip that isn't mentioned on the video is that the 13mm nut for the main wire to the alternator can be easily removed by an impact driver, once you unbolt the alternator and rotate it into position.

To reinstall an alternator on the bracket, I had to compress the threaded nut inserts back into the alternator bracket so they are flush with the bracket, but this was difficult to do which the alternator is still captive behind all the hoses. Eventually I remembered the trick of using the alternator bolt by running it in backwards and using a socket as a large washer. The writeup that I found in this X166 forum for voltage regulator is for a 2016 with a 6 cylinder, that guy said it was quick and easy. Note that his tool list is different than the M278 V8.
https://mbworld.org/forums/gl-class-...placement.html
I think the M278 V8 access to the alternator may be tighter than the V6, or maybe I'm just a bad mechanic. I have an innate skill to triple or quadruple the time for a repair.

TLDR: Red battery symbol on the dash caused by low running voltage was fixed by replacing the Voltage Regulator with the Valeo part from FCP Euro (it's the exact same part as OEM) without fully removing the alternator from the vehicle. My 3-year-old battery was fine.