Convert gml to topojson

What is the difference between .gml and .topojson?

Description: Geography data format created by Galdos Systems and supported by the Open Geospatial Consortium; used by geographers for storing geographical data in a standard interchangeable format.; TopoJSON is a file format for encoding geographic data structures, based on the GeoJSON format. It encodes geometries more efficiently and supports topology, which enables more accurate representations of geographic objects. TopoJSON eliminates redundancy and simplifies data, making it smaller and easier to work with.