Anywhere but on the battlefield, Grist is a Legendary Planeswalker Creature — Grist Insect. Once it enters the battlefield, it is no longer a creature and is just a planeswalker. Anything that could search for or affect a creature or planeswalker card in zones other than the battlefield could affect Grist. For example, you could put it onto the battlefield with Chord of Calling, it could be countered by Essence Scatter (but not by Negate), and opponents couldn't make you discard it with Duress.