What the US would gain is unclear, besides saving some contribution money.
What Trump gains is turning the narrative away from criticism of his leadership with regards to the Covid crisis and reframing it as something for which he doesn't have to take any responsibility because it was all China's and the WHO's fault.
Basically, the Covid story is here to stay. Make it about WHO shortcomings rather than Trump shortcomings.
Other presidents might have been wary of the reactions of US allies at picking a pointless fight with the WHO, especially when their expertise is much needed. Trump however has expended sofrequently indicated he doesn't care much diplomatic capitalabout what the rest of the world thinks and international goodwill alreadymany of his core supporters like that it really doesn't matter much by nowabout him.