An exclamation expressing approval, encouragement, or excitement (very informal and likely nonstandard).
Possibly dialectal or slang evolution of 'go you!' or a variant pronunciation; this appears to be rare or nonstandard usage without clear etymological documentation.
Words that seem made-up often come from rapid speech and dialect—'gayyou' might be what happens when 'go you!' gets compressed and repeated so fast in casual speech that it becomes a single exclamation, which is how lots of slang actually originates.
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