Here the word "gifts" refers to the sacrifices that the people made to the idols that they worshiped.
# they made a sacrifice of every firstborn of the womb
The phrase "firstborn of the womb" refers to the firstborn child of a woman. The word "sacrifice" can be translated with a verb. AT: "they sacrificed every woman's firstborn child" (See: [[rc://en/ta/man/translate/figs-explicit]])
It is implicit that they put them into the fire in order to offer them as burnt offerings to their idols. AT: "sacrificed them as burnt offerings" (See: [[rc://en/ta/man/translate/figs-explicit]])