So, if anyone is interested: I have a "solution" for the case that you don't resize the iPad screen while the playground is executing. Here is how it goes:
After setting your view as PlaygroundPage.current.liveView, make an UIView.animate with an dealy and duration of about 0.01. As animation, do something that the user can't see, like setting the alpha from a view from 0.0 to 0.01. Now, when the completion handler is entered, you will get the correct size of the view that you did set as your liveView, simply by calling yourView.frame.size. That's where you can set up your gesture recognizers and your SKScene.
I know, it's an ugly hack, but it works. Keep in mind, as soon as you resize the running playground in the app, the black bars reappear and the recognizer will stop working correctly. Still haven't found a solution for that - Constraints also don't solve that.
Just wanted to add here that I have been experiencing the same problem with SpriteKit. As apparent in the source code for several of Apple's playground books, there is a way to accomplish layout resizing using a compiled storybaord in a .storyboardc file. I have seen a few snippets that used programmatic constraints as well. The problem here is that there are no examples or documentation for creating dynamic layouts at all. I'd love to hear if anyone has seen anything along these lines on the internet...