。
She scrubbed the kitchen floor hard with a brush.
Literal
She [topic-は] kitchen [genitive-の] floor [object-を] brush [with-で] scrubbing washed.
ごしごし is a 擬音語 (sound mimetic) for the rough scrubbing or rubbing sound — used adverbially with cleaning verbs like 洗う, 拭く, or 磨く. Often written in katakana for vividness (ゴシゴシ). The で here is the instrumental 'with' marking the brush as the tool. ブラシ is a katakana loanword from English 'brush.'