Yes, a .gml file is just an xml file.
When you open a .gml file for the first time in Liquid Studio, you will be prompted to select the editor to use for the extension .gml.
Select the 'XML Document' and click OK. Whenever you open a .gml in future it will automatically be opened as an XML Document.
The .gml file should contain a schemaLocation attribute specifying the location of the GML schema. The .gml will be validated in Liquid Studio against the specified schema.
You can also open GML schema in Liquid Studio and use all of the schema tools such as generate sample gml documents, and generate documentation.