Depends on the Gammax 400. There's a slightly older design which came stock with pushpins for Intel. That was changed in the 400B/400R (blue/red led fans) to a variable mount as is the newer 400, which also includes an AM4 mount compatability.
So as long as you get new stock, you'll be good, if it's an old one, it'll not be good as it won't fit AM4, just am3+ and prior.
This is quite normal when a new series mobo is announced, sellers get stuck with older inventory that doesn't fit the new mobo mounts or web pages that don't get updated to say the mount is compatible. By now, it shouldn't be an issue, especially from major players like Amazon etc, but some back-county little mom&pop store might not be so lucky.