If you say "Cxu gxi veras" I think you have to refer to a specific object. "La Biblio, cxu gxi veras?" would work (according to my logic and what I read in earlier discussions, anyone who knows more Esperanto is welcome to correct me), but if you're just referring to something someone just said, I think the gxi should be omitted
This is the best I can find so far-
Really it can be seen as either a contraction between a verb and an adjective or "ver-" acting as a root, therefore being able to hold noun, adjective, adverb, and verb endings (vero, vera, vere, veras, etc.). I've seen it numerous times throughout the Esperanto course, particularly with the adjectives bongusta and bona.