ronaldp31, saw your message and hope it's okay to reply here.
Anyway, if you want to send me a message with your serial number, I can double-check to see what kind of M.2 SSD your motherboard supports or hopefully your local ASUS support or store can do this too.
If you're sure your system is only able to support SATA3 M.2 SSDs, the ASUS website only indicates that up to 256Gb SATA3 M.2 SSDs have been approved, but this just usually means we haven't bothered to approve larger sizes, so you may want to check it out for yourself and just be prepared if there are any problems.
https://www.asus.com/Laptops/ROG-G501JW/specifications/
You may have seen this post already, but one ROG member, akinto2006 under post #18 reports they could get a 525Gb SATA3 M.2 SSD working although it's not anything we could guarantee on the ASUS end, so just be aware of this (I may be worrying a bit too much).
https://rog.asus.com/forum/showthread.php?83477-G501JW-NVMe-M-2-SSD-Support/page2
If your system already came with an M.2 SSD installed though, it's a good idea to double-check which type of M.2 SSD your notebook supports though since I'm guessing ASUS may often want to bundle PCIe M.2 SSDs from the factory although this probably will not always be the case either. thanks.