I think the issue here is that dis_map is already a ggplot object and you're trying to add another with + ggplot(gdat). Try removing the ggplot(gdat) line and defining your aes() and data = arguments individually for each geom you add to the map layer.
I think the error is coming from geom_contour not having any data or aes mapping assigned to it. Because there is no 'global' ggplot data + mappings defined in this case, you need to provide them for every geom. You can then provide the appropriate aes mapping individually for each geom. In this case it's telling you than geom_point does not recognize the z aesthetic, but geom_contour requires it. So your code should look something like this:
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