。
She's my girlfriend.
Literal
She [topic-は] my girlfriend is.
ガールフレンド is the katakana loanword from English — used for romantic 'girlfriend.' Native Japanese has 彼女 ('she / girlfriend'), which is by far more common in everyday speech for the romantic sense; ガールフレンド sounds slightly Western, slightly formal-or-distancing, and can also mean simply 'a female friend' depending on speaker and context. The sentence is grammatically as basic as it gets — topic + complement + copula.