https://sl.wikipedia.org/wiki/Kaja -- as a Slovene name, this seems to be primarily treated as a short form of names beginning with "Ka-", such as Kajetana, Karla, Katarina. The derivation from Gaja seems to be an afterthought.
― Anonymous User 9/5/2020
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Also a girls name in Croatia. It was fairly common before 1970, and became popular again in the late 1990s.
In Croatia Kaja is almost always thought of as a diminutive of Katarina, but it can be a short form of any name beginning with "Ka" like Karolina. In fact very rarely it was also used as a masculine name, a variant/nickname of Kajo (Gajo, Gaj) itself the Croatian form of Gaius.