A gunshot / gunpowder residue test is only useful as evidence that somebody did shoot a gun if they are found with this residue on their hands.
But lack of such residue doesn't show that the people have not recently fired a gun because in many cases not enough gunshot residue is deposited on the skin to be picked up in those tests.
Years ago, some police recruits were tested immediately after they left the firing range doing a qualification shoot,
and a lot of them came up "negative" for GSR, although every single one had fired dozens of rounds.
But lack of such residue doesn't show that the people have not recently fired a gun because in many cases not enough gunshot residue is deposited on the skin to be picked up in those tests.
Years ago, some police recruits were tested immediately after they left the firing range doing a qualification shoot,
and a lot of them came up "negative" for GSR, although every single one had fired dozens of rounds.