You are invoking tall array's gather function, https://www.mathworks.com/help/matlab/ref/tall.gather.html
The syntax for that permits multiple outputs, but only when there are multiple inputs, in which case it gathers each input array into its corresponding output array.
gather() with multiple outputs cannot be used to collect and gather multiple outputs of a function: you have to assign those outputs to variables and gather() the variables.
But the fitcnb() function you are invoking is https://www.mathworks.com/help/stats/fitcnb.html which only ever has a single output anyhow. You can extract HyperparameterOptimizationResults as a property of the returned model, not as a separate output.
