The GH5 has a bigger, more powerful processor, and some of this extra resource is applied to noise reduction. There's all sorts of smart stuff going on here: some of it to do with edge detection and intelligently handling large areas that are the same or similar colours. (If you know that you're looking at the sky, it's unlikely that its going to be full of visible noise.)
RedShark Reader Andrew Dean has sent in this admittedly rather ad-hoc but extremely useful video comparing the low light performance of the GH5 and the GH4 - and the differences are striking.
Low light performance makes a huge difference to the usability and versatility of a camera.
It all makes the GH5 seem, yet again for the famous Panasonic mirrorless range, like a real powerhouse and an incredible bargain.