Code

What is a .gml file?

A .gml file is a Geography Markup Language. This is a code, script or executable format. Confusingly, this extension has been used by 3 different formats over the years — they're all listed below.

This extension has been used by more than one format

3 different formats have claimed .gml over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .gml file

VS Code opens and highlights nearly every text-based code format.
Never run a script you didn't write or trust.

The registry records this extension against Geography Markup Language, Geography Markup Language, CityGML File, so software from that family is the most likely match.

How to convert a .gml file

Text-based code needs no conversion — just save with a different extension if a tool insists.

MIME type

When a .gml file travels over the web, servers label it with this MIME type:

application/gml+xml

What's a MIME type?

The technical record

For archivists, developers and the merely curious: these are the PRONOM registry entries for .gml.