。
She's a kind-hearted girl.
Literal
She [topic-は] feeling [attributive-の] gentle girl is.
気持ちのやさしい女の子 has a small but interesting structure: 'gentle of heart girl,' built with 気持ち ('feeling, heart') as the modified attribute and やさしい ('gentle, kind') as the modifying adjective, joined by の. Japanese can express many 'X is Y' descriptions with this attributive-の pattern when it would feel cumbersome with full clauses. The formula '[noun]の[adjective][noun]' compresses 'a person whose [noun] is [adjective]': 髪の長い女性 ('a woman with long hair'), 心の優しい人 ('a kind-hearted person'). やさしい spans both 'gentle' and 'kind' — the distinction English splits, Japanese fuses.