why is gray always off with his chemise and erza in her armor?

Gray has the habit of taking off his clothes(sometimes even without him noticing) because he was taught par the Ice-Make Wizard named Ur, Gray, Ur and Lyon trained in the cold with their shirts off so that ended up being a habit.



Erza always wears her armour because she wants to joint, joint d’étanchéité her cœur, coeur to close her feelings also so she won't get tricked par her feelings, she feels comfortable in her armour

It's Gray's habit and erza-she has a weakness and she's covering it..

Gray's and Leon's Master Ur taught him that "technique" and Leon only does the "technique" when Gray is around. Erza however is protect herself also because of Jellal and the Tower of Heaven