PostGIS window function not picking up index?
A query reading the whole table will not profit from the index. It's cheaper to read the whole table sequentially.

The spatial GiST index you display can be useful for other types of queries filtering a small percentage from the table, like:

SELECT * FROM point_data WHERE ST_DWithin($input_geometry, geom, 5000);


Understanding the decisions of the Postgres query planner requires intimate knowledge of Postgres. Start by reading this chapter of the manual and follow links from there:


How the Planner Uses Statistics


And more particularly, about spatial indexes in the PostGIS manual:


Taking Advantage of Indexes