noun, na-adjective
Describes a disorderly mixture of various things, often with a negative or chaotic nuance. Can be used as a noun or adjectival noun (with な).
机の上は本や書類でごたまぜだ。
The desk is a jumble of books and papers.
いろんな意見がごたまぜになって、会議は混乱した。
Various opinions got all mixed up, and the meeting became chaotic.
混ざる is a verb meaning 'to be mixed', whereas ごたまぜ is a noun/adjectival noun describing the resulting state of being jumbled.
Likely a compound of ごた (onomatopoeic element suggesting disorder) and 混ぜ (from 混ぜる, 'to mix'). The exact derivation is uncertain.