Clients and tools that support GML Read and/or Write
The following is an incomplete list of known clients and toolkits that support the ability to at a minimum ingest a GML file and display the contents of that GML file. If anyone knows of other clients (free or for fee), please let us know. Finally, just to be clear, any transactional Web Feature Service (WFS) reads GML and any compliant WFS writes GML. So clients that support a WFS interface most likely also support some level of processing of GML files. Please note that the following list is not an enummeration of server products that support GML. This list can be found on the OGC Implementing Products page. You may also refine your search of implementing products by going to "View By Specification".
Carbon Tools GAIA - Gaia 3 is the Windows application for accessing, visualizing and sharing location content. Gaia 3 lets you seamlessly access and use a vast array of location content and services from your Windows desktop. Read and display GML files. Client. Free.
gvSIG 1.1 Supports WFS 1.1 and the ability to read and write GML 3.1.1. Client. Free.
Snowflake GML Viewer . Reads any GML 2 or GML 3 application schemas and more. Client. Free
uDig - User-friendly Desktop Internet GIS (uDig) is an open source spatial data viewer/editor, with special emphasis on the OpenGIS standards for internet GIS, the Web Map Server and Web Feature Server standards. uDig provides a common Java platform for building spatial applications with open source components. Client. Free.