Ah, I forgot that this diesn’t work directly in Python.
You need to cast the gcp first.
Do this once at the beginning of your script:
Placemark = jpy.get_type('org.esa.snap.core.datamodel.Placemark')
Then you can later do:
node = gcp_group.get(i)
gcp = snappy.jpy.cast(node, Placemark)
gp = gcp.getGeoPos()