Picture something light and bright: a voice that goes up at the end, a hand , a row of red and yellow flags moving in the wind. Nothing is heavy, nothing is being weighed. That lightness is what gaily always adds, whatever it is attached to — it never tells you what happened, only how light it looked or sounded. Where the lightness lands is what makes the three uses different: on a person who is happy, on a person who should have been paying attention, or on a thing covered in strong colours.
From the word’s history
Wiktionary records gaily as gay + -ly, where -ly turns an adjective into an adverb. The older adjective covered both being cheerful and being bright to look at, and those two lines match the two halves of the word today.
Wiktionary 記的來源是 gay 加上 -ly，-ly 是把形容詞變成副詞的字尾。早期的 gay 同時有「快樂的」和「鮮亮的」兩面，而今天 gaily 的兩組用法正好對上這兩條線：一邊講心情，一邊講顏色。
Wiktionary (kaikki) gaily adverb entry: From gay + -ly. (source_ref kaikki:3f6f69a190036ae4:643509064:9342)
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