、。
She expressed her gratitude for the gift.
Literal
She [topic-は] gift [toward-に対する] gratitude [genitive-の] feeling [object-を] expressed.
に対する is the attributive form of に対して ('toward, with respect to') — used to modify a following noun. ~に対する感謝 ('gratitude toward / for X') is standard. 表わす ('express, show, manifest') in the alternative spelling 表わす is slightly older / written-style; 表す is more common in modern writing. 感謝の気持ち ('feelings of gratitude') is one of the formulaic phrases for thanking someone in formal register, common in thank-you cards and ceremonial speech.