I found a little trick on how Outlook retrieves mail from Gmail and sort the labels into sub-folders. For that I created a [Gmail] label and nested every other label underneath.
It works for some labels - they show up in Outlook as a neat subfolder including the tagged mails. But others are just getting reverted to their old position, so instead of staying nested under [Gmail], they end up back on the same hierarchy level and thus don't show up in Outlook at all.
There seems no pattern to which labels stay and which get reverted either.