Greetings,
You haven't given us much to go on.
You've got a z270MX-gaming 5 correct?
What CPU and RAM?
Is the RAM on the QVL for the board?
Will it run at full speed with XMP disabled, or just the SPD detected speed?
If not, have you tried setting the values manually in M.I.T?
What BIOS rev are you running?
You board appears to be behind in BIOS releases (latest is F6D). I seem to recall F8 as the BIOS that fixed this issue for others (don't remember exactly).
I checked the EU site as well, but the G5 there only has F6D as well.
I recommend you contact Gigabyte support for assistance and resolution. I think you will need to later BIOS rev.