。
It's the kind of gesture where her ponytail flips and you can't help but stare.
Literal
Ponytail [subj-が] flipping, involuntarily become-entranced gesture [copula-だ].
A single sentence where the entire main predicate is just 仕草だ 'it's a gesture,' and everything before it is a relative clause modifying 仕草. 翻る (ひるがえる) is the verb for fabric, hair, or flags fluttering / flipping in motion. 思わず is an adverb meaning 'involuntarily / in spite of oneself,' often paired with verbs of perception or reaction. 見とれる is a compound of 見る (see) + 惚れる (fall for), literally 'fall while looking' = 'to be entranced watching.'