、。
Was the gate wide open, or only slightly open?
Literal
Gate [topic-は] greatly was-open [or-か], otherwise just-slightly was-open [Q-か].
それとも ('or (instead)') is the alternative conjunction for disjunctive questions ('was it X or Y?'). Distinct from the listing particle や or the conditional ~か. ほんの ('just, merely, only a') is a diminutive modifier — ほんの少し ('just a little'). 開く ('open') here conjugated as あいていた ('was in an opened state').