I purchased the same sound card just because I hadn't used a dedicated sound card in several years, I have used it on a X Hero and currently on a X Apex, I have zero issues as far as initialization, I use it with Logitech Z-906 5.1 speaker system, I prefer speakers to headphones.
I don't believe changing boot settings are going to change anything (my boot settings are default), if you use a nvme boot drive Legacy install is not an option and shouldn't make any difference, as to why you have issues on the ASUS and not the Gigabyte I don't have an answer
Do you have the power cable connected? the card works without the power connector as it just supplies power for the RGB LED's
How long have you owned the card, can it be returned to seller for a replacement?
Is your bios settings all default, any overclock, specifically a blck overclock? are the pcie settings default, as you didn't manually change them from auto to Gen. 3.0, 2.0 etc.