Having been turned into a gel or gel-like substance; congealed, solidified, or set into a firm jelly-like state.
Past tense and past participle of gel (verb), from gelatin; the -ed suffix indicates a completed action or resulting state.
A 'gelled' substance shows how English uses the same word for both the process and the result—your hair is 'gelled' after you apply gel, using the verb as an adjective!
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