The following is unfortunately completely undocumented and therefore to be used at your own risk. It may stop working in a future version of matlab or may not even work in your version if you're not using R2020a which is where I've tested it:
It's a shame it's completely undocumented since mathworks have gone to the trouble of building this very complex machinery to completely change the way gather (and other functions) outputs progress.
Basically (sic! it's actually very complex with closures, futures, promises, lazy evaluators, executors, etc.) at some point gather will ask the abstract class ProgressReporter for the current progress reporter. By default it's a CommandWindowProgressReporter which is what displays the text to the command window. You can override that default with any other class derived from ProgressReporter and matlab provides such a class NullProgressReporter that outputs nothing.
It's a much better solution than using evalc and really mathworks should move it out of the internal package and document it.
Note: to go back to the default:
You can also create your own ProgressReporter to customise the progress display to your liking.