User blog comment:B214/Volume 21 info & spoilers/@comment-95.90.227.186-20160507145425/@comment-24.113.252.213-20160507175737

Sacred Gears can only be "natraly" possessed by humans (Issei, Asia, Kiba) and human hybrids (Vali). Sacred Gears can me possessed by supernatural beings who aren't in any way human. This includes Angels, Fallen Angels (Raynere is a good example), and devils (Sairaorg is an unofficial possessor).

I think it is possible for Issei to possess the Boosted Gear even if he was a complete dragon down to his soul. For 3 reasons:

1) The Boosted Gear is dragonic in nature, so he'll have an affinity for it.

2) God is dead so there are bugs in the Sacred Gear System that no one knows how to fix.

3) We've been shown by Raynere, in the light novels and anime, that Sacred Gears can be transfered to other beings, whether they're human, non-human.