adverb, adverb taking the 'to' particle, noun or participle which takes 'suru'
scrubbing hard; scrubbing clean
Mimetic adverb describing vigorous scrubbing or rubbing, often with a brush or cloth. Can be used adverbially with と or as a する verb.
鍋をゴシゴシ洗った。
I scrubbed the pot hard.
汚れをゴシゴシこすって落とした。
I scrubbed off the dirt by rubbing hard.