Once, but long ago, according to Online Etymology it started off like this:
gay
1178, "full of joy or mirth," from O.Fr. gai "gay, merry," perhaps from Frank. *gahi (cf. O.H.G. wahi "pretty"). Meaning "brilliant, showy" is from c.1300.

From the time it became showy it may have gone downhill.
Then I will use merry or joyful.