Gaelic zodiac signs aren't what you think they are. There isn't a group of zodiac signs that specifically fall under the group “Gaelic.” There isn't even Gaelic Astrology.
The closest thing we have to Gaelic Astrology is Celtic Astrology. Since the Celts spoke a Gaelic language, Celtic Astrology probably is the proper term for what you think of as Gaelic Astrology. That would mean Celtic zodiac signs are Gaelic zodiac signs.
But what we know of Celtic, or Gaelic, Astrology is really quite limited.
The Celts had an oral tradition, meaning everything was passed down verbally. While they did have a written alphabet called the Ogham, very little was written down and no one remembers how to read it. Anything we have written by Celts using the Ogham – we can't read it.
We know the Celts used astrology, but we don't know much about it.
Writings by the Romans mention that the Celts read the stars and had advanced mathematics that they used for astrology, and were apparently quite good at astrology, but that's about it. Any other information about Celtic or Gaelic Astrology, that may have been written down was lost over the years, much of it destroyed. That includes anything about Celtic or Gaelic zodiac signs.
So what exactly are Gaelic zodiac signs?
Gaelic zodiac signs don't exist.
Celtic zodiac signs don't exist – at least as the Celts knew them.
Much of what we call Celtic Astrology or Gaelic zodiac signs today was created to fulfill the sudden interest in all things Celtic, Gaelic and Irish.
Druidism became popular.
Stonehenge became popular.
But is any of it true?
If you want it to be.
If you want to believe in Celtic Astrology, it's true.
If you want to believe in Gaelic zodiac signs, they're real.
Maybe one day we'll know more about Celtic Astrology. We might even discover that there really are Gaelic zodiac signs. But until then, your guess is as good as mine.