The nappy split and now there seems to be a funny gel like substance on my baby's skin. What is it and is it harmful?

The gel you can see is what we call SAP - super absorbent polymer! As its name indicates, it's the super absorbent ingredient that helps absorb the liquid and keep your baby dry. It's non-toxic and poses no threat to your babies skin. It's best to simply wipe the gel off and dispose of it in the bin.