The word at issue is "גּד (gad)" and whether it is the Hebrew word meaning "troop" (H1410) or the name of a foreign god meaning "fortune" (H1409) (Brown-Driver-Briggs' Hebrew Definitions). Most modern translations translate the phrase along the lines of "good fortune has come". While it is possible that Leah considered the birth of Gad as a sign of good fortune (if she payed tribute to such a pagan notion), the interpretation of "A troop cometh" fits more squarely within the context for two reasons.
"And Leah said, A troop cometh: and she called his name Gad." (Genesis 30:11, KJV)
First, Jacob's prophecy concerning Gad in Genesis 49:19 plays on the pun of "Gad" meaning "troop". Jacob says, "Gad, a troop shall overcome him: but he shall overcome at the last" (Genesis 49:19). The NIV, ESV, and NASB all substitute "raiders" for "troop", but the meaning is similar. The Genesis 49:19 prophecy demonstrates that Gad's name is associated with "troop" (or any large company such as raiders) rather than "fortune".
Second, the name of each of Jacob's sons born in the sequence from Genesis 29-30 has a meaning based on the circumstances surrounding the birth (not including Benjamin, who was born later in Genesis 35). These names, their meaning in Hebrew, and the surrounding circumstances are as follows (the KJV margin indicates the meaning of each name):
- Reuben (See a son): "And Leah conceived, and bare a son, and she called his name Reuben: for she said, Surely the LORD hath looked upon my affliction; now therefore my husband will love me." (Genesis 29:32)
- Simeon (Hearing): "And she conceived again, and bare a son; and said, Because the LORD hath heard that I was hated, he hath therefore given me this son also: and she called his name Simeon." (Genesis 29:33)
- Levi (Joined): "And she conceived again, and bare a son; and said, Now this time will my husband be joined unto me, because I have born him three sons: therefore was his name called Levi." (Genesis 29:34)
- Judah (Praise): "And she conceived again, and bare a son: and she said, Now will I praise the LORD: therefore she called his name Judah; and left bearing." (Genesis 29:35)
- Dan (Judging): "And Rachel said, God hath judged me, and hath also heard my voice, and hath given me a son: therefore called she his name Dan." (Genesis 30:6)
- Naphtali (My wrestling): "And Rachel said, With great wrestlings have I wrestled with my sister, and I have prevailed: and she called his name Naphtali." (Genesis 30:8)
- Gad (A troop, or, company): "When Leah saw that she had left bearing, she took Zilpah her maid, and gave her Jacob to wife. And Zilpah Leah's maid bare Jacob a son. And Leah said, A troop cometh: and she called his name Gad." (Genesis 30:9-11)
- Asher (Happy): "And Leah said, Happy am I, for the daughters will call me blessed: and she called his name Asher." (Genesis 30:13)
- Issachar (A hire): "And Leah said, God hath given me my hire, because I have given my maiden to my husband: and she called his name Issachar." (Genesis 30:18)
- Zebulun (Dwelling): "And Leah said, God hath endued me with a good dowry; now will my husband dwell with me, because I have born him six sons: and she called his name Zebulun." (Genesis 30:20)
Each of these names reflect the surrounding context of the birth. As for Gad, the context is that Leah was no longer able to give birth so her maid Zilpah gave birth instead. The significance of this is that more children are expected to come from Leah. This is what is meant by "a troop cometh".
- Joseph (Adding): "And God remembered Rachel, and God hearkened to her, and opened her womb. And she conceived, and bare a son; and said, God hath taken away my reproach: And she called his name Joseph; and said, The LORD shall add to me another son." (Genesis 30:22-24)