Bushiroad has said that this promo can be discarded as almost any Grade in the game.
V-PR/0311 Curiophase Dragon
Grade 1 / Normal Unit / Dark Zone – Gear Chronicle – Gear Dragon
CONT [Hand] When this card would be discarded, it may be treated as Grade 4, Grade 3, Grade 2, or Grade 0.
AUTO (RC) When it boosts, one of your Grade 4 vanguards gets Power+5000 until end of turn.