Charles Ellicott Commentary Numbers 26:18

"These are the families of the sons of Gad according to those that were numbered of them, forty thousand and five hundred." — Numbers 26:18 (ASV)

Forty thousand and five hundred.— This shows a decrease of 5,150. Reuben, Simeon, and Gad encamped together on the south of the Tabernacle (Numbers 2:10), and had probably been mutually contaminated by each other’s evil example.