。
Knock and the door shall be opened.
Literal
Gate [obj-を] knock-[cond-ば] knock person [subj-は] it-will-be-opened-[honorific-もらえる][probably-だろう].
A Biblical-sounding maxim (from Matthew 7:7), rendered into Japanese with classical-ish grammar. ~ば conditional, the pattern X は Y てもらえる (passive/receive of benefit: 'X will have it done for them'), and the hedged future だろう. 門 is treated as a door to be knocked on, not a gate to be entered — a translation choice reflecting the original's metaphor.