"i have gaelic" is quite a literal translation of "tha gàidhlig agam," right? i would use this to mean "i know gaelic."
Yes, the literal meaning is basically I have Gaelic and the intended one is I speak Gaelic, I know Gaelic. Apparently that’s how people in Scotland happen to say it in English too. :)
Not everybody in Scotland uses that terminology. Some, yes, but it isn't widespread.