ELI5: What is the effect of removing government spending from GDP?

Commerce Secretary Lutnik in the U.S. intends to separate government spending from GDP calculations. Economists are worried about how this will impact the markets. What is the logic behind removing government spending from the calculation, and why would it impact the markets? How would it impact domestic and international economies? This is far from my area of expertise and while I can read about it, having some foundational understanding would really improve my ability to find the nuances and articulate them.