"As a user of Proton Mail, I should be able to exclude the results of. my search by one or more labels I select, in order to select all non-matching messages in a folder."

This would let me move, delete, or otherwise select all messages without a particular label, rather than only using the positive search match that I now see.

Ideally, the filter would be something like this:

!"Gmail import" or -label:"Gmail import"