Retailers have already been solicited for preorders for the miniatures line, and the gargantuan promo is part of it, but we haven't been told what figure it is.
BTW, the dates for the figures coincide with the rules releases. The 6-figure starter will release with the D&D Starter rules, July 15 for mass market, and July 3 for WotC Premiere stores. The booster set will release the same as the PHB, Aug19 for mass market, Aug 8 for Premiere.