Sam2299, thanks for the link and yes, it appears to only apply to our motherboards that are sold separately and not the motherboard inside the GL12CX.
At the end of the article, there are some links for the related motherboards.
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Not sure if everyone at ASUS will agree with this or if it's correct, but guessing that it is assumed most customers who buy desktops complete like the GL12CX would not normally want to spend much time tinkering with the system and would prefer to just install and play games, so there are not as many features or options for customers to adjust compared to our motherboards sold separately.
Unfortunately, not able to check a GL12CX system right now if you want to check with more people or check back, but expect the bios to be more limited as you may have found already although you may want to check if there are any settings to help with this.
Otherwise, if you haven't looked through the Armoury Crate software in Windows as mentioned at our global website below, you may want to check that too and experiment with the 'manual' mode.
Not sure if your GL12CX came with the air-cooled or liquid-cooled cpu, but just a reminder that you will see some performance issues if there is overheating, so you also want to make sure you have adequate cooling.
https://www.asus.com/ROG-Republic-Of-Gamers/ROG-Strix-GL12/Features/
"Exclusive Software
Unified System Tuning and Personalization
ROG Armoury Crate puts all essential system and lighting control at your fingertips in a single utility. You can easily change settings for both and define up to four game profiles that automatically make adjustments when you launch your favorite titles.
Armoury Crate even lets you manually overclock ROG Strix GL12CX beyond the processor’s factory-overclocked speeds. These options are just the beginning for what will become an even more versatile tool."