polygon is a property of CollisionPolygon2D. As shown in the linked doc, its type is PoolVector2Array. This is an array of
Vector2 objects representing the vertices of the polygon. For example to set a triangular shape, you need three points:
var points = PoolVector2Array([Vector2(0, 0), Vector2(0, 100),
borderCol.polygon = points
Note that in GDScript, there's no need for
get()/set() - you can just access the property directly to get or set the value.