The woman standing by the gate is a famous singer.
Literal
Gate [gen-の] side [at-に] is-standing-[ている] woman [topic-は] famous singer [copula-だ].
そば ('side, next to') is a locational noun similar to となり, 横, 近く but specifically meaning 'beside, next to.' 立っている marks the resulting state of standing. The relative clause 門のそばに立っている modifies 女の人.