Grammarist is not a dictionary-approved word. It specifically relates to this website.
The term for a person who specializes in grammar is a grammarian. Also, someone who creates or standardizes a syntactical construct is a grammarian.
The word grammar has largely stayed unchanged from its Latin root of gramatica.
As you can see by this n-gram, the usage of grammarian has dwindled over time.
Side note: in the French version, grammairien, the word used to also carry the meaning of magician.