。
She gasped.
Literal
She [topic-は] breath [object-を] swallowed.
息を飲む (also 息を呑む) is the partner idiom to 息を殺す — literally 'swallow one's breath,' meaning 'gasp, have one's breath taken away, be left breathless (in surprise, wonder, or shock).' This is the involuntary, reactive version: an involuntary intake of breath at something striking. Common in literary descriptions: 美しさに息を飲む ('be left breathless by the beauty'), 恐怖に息を飲む ('gasp in horror').