Will this RAM work with a Ryzen 7 5800x?
GSkill DDR4 4000
It should work but I have seen reports of problems getting the infinity fabric clock to 2000MHz on the new chips.
As soon as you post your Computer specs especially Make & Model of your Motherboard I can check to see if your RAM is listed as being compatible and at what speeds (OC).
GSkill QVL only lists Intel boards, i would post a query with their support/forum as well as checking the mobo QVL.
Goodplay it does seem to be compatible with Intel Motherboards and not Ryzen motherboards.
Also went to G-Skill RAM Calculator and the highest Ryzen Motherboard RAM speed for Ryzen was 3800 Mhz .
While Intel Motherboards had RAM that was 4000 Mhz and higher being compatible.
Don't know if you use that RAM if it will automatically slow down to the fastest speed the motherboard will support or not.
I would follow Goodplay suggestion and open a ticket with G-Skill or your check your Motherboard's RAM QVL list for your processor.
Note: As an example only, I picked Asus ROG Crosshair VIII Hero Motherboard and took at look at its QVL list for RAM. It does list 4000 Mhz RAM modules as being compatible: