。
She was happy about the gift.
Literal
She [topic-は] that gift [object-を] was-glad.
喜ぶ ('to be happy/glad') is an emotion verb that takes the source of happiness with を — unusual from an English perspective, where 'be happy about' takes a preposition. Grammatically, Japanese treats 喜ぶ as an action ('to do/show gladness'), so the source of joy is its object. The related adjective 嬉しい instead uses が or は for the cause: 贈り物が嬉しい ('the gift makes me happy'). The two together (verb + adjective) form one of Japan's classic adjective-verb emotion pairs.