The old property never worked as expected because it was impossible expose the data as props.
Now, there are two methods to handle this: weight_get and weight_set
Example use:
import bpy
ob = bpy.context.active_object
gpd = ob.data
gps = gpd.layers[0].frames[0].strokes[0]
i = 0
print("Weights\n================================")
for pt in gps.points:
gps.points.weight_set(vertex_group_index=0, point_index=i, weight=0.5)
i +=1
i = 0
for pt in gps.points:
weight = gps.points.weight_get(vertex_group_index=0, point_index=i)
print(weight)
i +=1