So I'll start saying that I have the version GL502VSK with 7th gen processor and same specs, but it comes with the 230w adapter! It isn't indeed immune to this problem and just some days ago I've started reading this post and many others. According to an article I've found, an investigation is undergoing by those guys here, sorry for "guys" but I dunno how to name them at the moment. They're even proposing a class action due to all these facts.
http://www.classlawdc.com/2017/08/30/asus-gl502vs-gaming-laptop-battery-drain-defect-investigation/In addition to this, I've found many interesting datas from my own test, where I saw when playing at 60fps or less, actually the battery level waas increasing, strange. After tons of other test I've finally arrived to the solution and crearly say that probably in the production, design, project, they didn't calculate the power drain of the monitor with G-SYNC active and at 120Hz. With the help of another detailed post, this is explained with a well illustrated graph, that how much this guy could test with the different hz level of his desktop monitor, monitoring an increasing from 78w to 200W with the full 155HZ. This is not the case ye because it's only 120Hz, but after many sessions, I've set my monitor to 60Hz, and the result was clearly different, with the battery levels going up again. Even disabling the vsync and let the NB go to more than 200fps, for this example I was using H1Z1, the result was always an increase in battery level. When switching again to 120hz, it went down again.
Here the tests of Desktop monitor I've mentioned:
https://www.pcper.com/news/Graphics-Cards/Testing-GPU-Power-Draw-Increased-Refresh-Rates-using-ASUS-...I was so confused before those tests, because I was testing CPU and GPU on stress tests at maximum performance, but the battery wasn't going down at all, and that was too much strange that I couldn't reprocude. This has brought me there, to check monitor. Probably they didn't checked it with an application which was using correctly the 120Hz refresh rate and G-SYNC ON. This should be the answer to atleast the why it has so much power drain. Test it out pls and let me know what you think and your results. Currently I have an undergoing complain with Asus and still waiting for an answer due to this problem with the NB, the complain is also because this will be the 8th Notebook that I'll send them back. It's such ashame really.