This month's HJ Magazine confirmed what we've been expecting for a while!
The text says something like this:
"クイーンズブレイド刊行10周年を記念し、"クイーンズブレイド グリムワル" のアニメ化が決!! 作品はOVA全2巻形式で、今秋以降発売の対戦型ビジュアルブックに同梱し連続リリース予定とのこと。 さらによしもときんじ＆りんしんをはじめ、QB第1期アニメスタッフが再結集. 過去シリーズの美闘士たちもゲスト登場！新世代美闘士たちの華麗なる活躍を見すな!!"
Which would translate to something similar to:
"To commemorate QB's 10 anniversary, QBG's anime was greenlit! It'll be comprised of two volumes, bundled after the next gaming visual book to be released this fall. Not only we'll have Yoshimoto Kinji & Rin Shin, but also the anime staff of QB's first season. A beautiful fighter from the past series will make a guest appearance! Don't miss the debut of the next generation of beautiful fighters!!"
The wording is a bit funky so it's not 100% accurate, but after (or bundled with?) a quite likely Despina's book in Autumn, there's no doubt we'll be getting at least 2 Grimoire OVAs.