Document

What is a .gltf file?

A .gltf file is a GL Transmission Format (Text) (version 1.x). This is a document format, so it holds text and layout meant to be read. Confusingly, this extension has been used by 2 different formats over the years — they're all listed below.

This extension has been used by more than one format

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

How to open a .gltf file

Try LibreOffice (free, Windows/Mac/Linux) — it opens an enormous range of current and historic document formats.
Microsoft Word and Google Docs handle most mainstream document types.
If nothing opens it, open it in a text editor: many older document formats have readable text inside.

The registry records this extension against GL Transmission Format (Text), GL Transmission Format (Text), so software from that family is the most likely match.

How to convert a .gltf file

Once it opens in LibreOffice or Word, Save As .docx or .pdf to get it into a format you can use anywhere.

MIME type

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

application/json

What's a MIME type?

The technical record

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