So, the product page is not really clear about that https://www.asus.com/us/Tower-PCs/ROG-G20CB/ you could ask the same from their customer service. If you do not need the HDD anymore you can replace it by a SATA SSD such as: https://pcpartpicker.com/product/RbvZxr/samsung-internal-hard-drive-mz75e1t0bam
but opening the cabinet may cause warranty issues.
So, the product page is not really clear about that https://www.asus.com/us/Tower-PCs/ROG-G20CB/ you could ask the same from their customer service. If you do not need the HDD anymore you can replace it by a SATA SSD such as: https://pcpartpicker.com/product/RbvZxr/samsung-internal-hard-drive-mz75e1t0bam
but opening the cabinet may cause warranty issues.