John Kirk Robertson, D.D. wrote in his book The Aquarian Bible Of The Cosmic Christ (ISBN 0-9543605-1-6) that when you read in Hebrew Gad from left to right you get Dag, which means in Hebrew Fish or Pisces.
Diagram is courtesy of the author John Kirk Robertson, D.D.
John Kirk Robertson identifies:
1. Dan as Scorpio
2. Judah as Leo
3. Reuben as Aquarius
4. Ephraim (Manasseh) as Taurus
5. Asher as Virgo
6. Issachar as Capricorn
7. Simeon and Levy as Gemini
8. Naphtali as Cancer
9. Zebulon as Libra
10. Gad as Pisces
11. Benjamin as Aries
12. Joseph as Sagittarius
12. Joseph as Sagittarius
I found another source that contradicts the above mentioned identification, where Naphtali is identifies as pisces while, of course, all the other tribes have other identifications:
The standards of the tribes corresponded to the zodiacal signs of the constellations, so that in the east was the standard of Judah, with Issachar and Zebulun beside it, these three being opposite Aries, Taurus, and Gemini; in the south was the standard of Reuben, with Simeon and Gad, opposite Cancer, Leo, and Virgo; in the west was the standard of Ephraim, with Manasseh and Benjamin, opposite Libra, Scorpio, and Sagittarius; and in the north was the standard of Dan, with Asher and Naphtali, opposite Capricornus, Aquarius, and Pisces
From Yalkut Shimony, Exodus, 40:418 translated by Joseph Jacobs and Judah David Eisenstein Jewish Encyclopedia entry: zodiac
Yalkut Shimony is dated to the 13th century, but it contains sources that date to much earlier centuries. It is not clear when was written the paragraph quoted above.