I have just started playing around with Filters in Gmail. This is one thing that took me some time to figure out, how to filter by label. It turns out it was quite easy, it seems you can use any search string in the “Has the words” field of your filter. To get a label you simply put “label:your-label-name” in there.

You will get a warning: “Warning: Filter searches containing “label:”, “in:”, or “is:” criteria are not recommended, as these criteria will never match incoming mail.”
I think this makes sense as incoming email would not have a label.

More Gmail search options can be found here