John Calvin Commentary Genesis 30:9

"When Leah saw that she had left off bearing, she took Zilpah her handmaid, and gave her to Jacob to wife." — Genesis 30:9 (ASV)

When Leah saw that she had stopped bearing. Moses returns to Leah, who, not content with four sons, devised a method by which she could always retain her superior rank; and therefore, she also, in turn, substitutes her maid in her place. And truly, Rachel deserved such a reward for her perverse design, since she, desiring to snatch victory from her sister, did not consider that the same contrivance to which she had resorted could quickly be used against herself.

Yet Leah sins even more grievously by using wicked and unjust means in the contest. Within a short period, she had experienced the wonderful blessing of God; and now, because she stopped bearing for a little while, she despairs about the future, as if she had never participated in Divine favor. If her desire was strong, why did she not resort to the fountain of blessing? Therefore, in obtruding her maid, she gave proof not only of impatience but also of distrust; because faith is also extinguished in her heart, and with it, the remembrance of Divine mercy. And we know that all who rely upon the Lord are so tranquil and composed in their mind that they patiently wait for what He is about to give. And it is the just punishment of unbelief when anyone stumbles through excessive haste. All the more, then, should we beware of the assaults of the flesh, if we desire to maintain a right course.

Regarding the name Gad, this passage is explained in various ways by commentators. They agree on this point: that בגד (bagad) means the same as if Leah had said, “The time of bearing has come.” But some suppose גד (Gad) to be the prosperous star Jupiter; others, Mercury; others, good fortune.

They cite Isaiah 65:11, where it is written, they offer a libation to Gad. But the context of the Prophet shows that this should rather be understood as referring to the host of heaven, or of the number of false gods; because it immediately follows that they offer sacrifices to the stars and furnish tables for a multitude of gods. The punishment is then added, that as they had fabricated an immense number of deities, so God will number them to the sword. Regarding the present passage, it is highly improbable that Leah would extol the planet Jupiter instead of God, since she, at least, maintained the principle that the propagation of the human race flows from God alone.

I also wonder that interpreters understand this as meaning prosperous fortune, when Moses afterwards, in Genesis 49:19, leads us to an opposite meaning. For the allusion he there makes would be inappropriate, Gad, a troop shall overcome him, etc., unless Leah’s design had been to congratulate herself on the troop of her children.

For since she had so far surpassed her sister, she declares that she has children in great abundance. When she proclaims herself happy with her sixth son, it again shows the great esteem in which fertility was then held. And certainly, it is a great honor when God confers on mortals the sacred title of parents, and through them propagates the human race formed after His own image.