Does everything have a Gold and Diamond version?

Every creature has both a Gold and Diamond version, except for the "Unique" creatures, which have Gold but no Diamond versions.

Items sometimes have Gold versions, sometimes Gold and Diamond, or sometimes neither. You can look at an item's entry in the Discovery Journal to see if it has Gold or Diamond versions, which will be denoted by a Gold and Diamond button in the lower-right of its card (and you can press these buttons to see those versions, if you've discovered them).