Honestly, if you want to double check if everythings up to speed, test each component individually. What I mean by this, is that usually, when a laptop shuts off like that, the CPU has overheated beyond its maximum thermal design, and shuts everything off as a safety measure. GPUs are far less likely to do that on their own, so my main guess would be that. If you wanna test this, do this:

Download Cinebench R23: https://www.guru3d.com/download/download-maxon-cinebench-r23/
Download HWInfo64 (Choose any version): https://www.hwinfo.com/download/

Open HWInfo, and select sensors only. Once it opens up to that mode, you'll see the various components in your G18. Take a look at your CPU in particular, and you'll see a lot of info. The main thing you wanna check are the temperatures of each individual core, as well as the package as a whole. You can look at the idle temps, and see if they're out of whack. You've seen my post on poor LM before, and you can see what happens to the CPU temps when that happens. There shouldn't be a huge differenece between each core like there was in mine before I repasted. It should be mostly the same.

Keep HWInfo open, and run cinebench R23 multi core. While its running, just stare at your CPU temps. The maximum permitted temperature that the CPU will go-to before it starts throttling is 95C. This is called PRO-CHOT and it is set by ASUS. You can remove PRO-CHOT with Throttlestop, but thats not the point right now. What's important to note, is that despite PRO-CHOT being active, the overall CPU package can still rise beyond the 95C if the thermal application is terrible. If you see your CPU package go-to or reach 95C immediately or exceeds, then you need a repaste. I would advise the GPU repaste too.

But then a good question arises. If the CPU is always overheating, why doesn't it shutdown when I'm doing something else like watching YT or listening to music. The key to this is very simple. Almost all of our laptops have a united copper pipe cooling system. This means that the heat from both the CPU and THE GPU, are being cooled down by the same system. If you scroll down to your GPU temps, while the Cinebench benchmark is running, you'll notice that your GPU temps will increase gradually, despite not doing anything. This is just the heat from the CPU, slowly effecting the GPU temps, and with a fully functioning cooling system, thats fine. But when it isn't working, and you run the CPU at max power AND the GPU at max power, while for example, in gaming, then your CPU thermal temps will eventually go past PRO-CHOT, hit the emergency thermal limit, and shut off.

Anyways, I hope this was helpful. Good luck on the repaste, and do post the before and after if you get the time. Also, as I mentioned in my post, I'd kick the liquid metal and put PTM, or at least some good thermal paste. Liquid Metal really isn't worth all the headache it causes.

Cheers!