Assuming the nectarine was ripe I'd guess the seed is infected with something. Gel in peach/nectarine can be gummosis. Usually seen on the trunk but sometimes affects seed inside fruit.
Certainly not worth trying to germinate if it is not solid.
Pretty early in the season for ripe nectarines. Still all hard here but some in stores coming from tropical QLD already. You may have fruit coming from North Island orchards or shipped from QLD.
Stone fruit are often picked before ripe to cope with shipping then allowed to ripen (soften?) afterward so the seed may not even be mature yet.
You will probably find better seed later in the season when fruit is properly mature.