I am interested in publishing a GML file as a WFS. I don't want to use a database like PostgreSQL. What would be the rough steps I need to do for that? I like to use open source software (geoserver). I don't have much experience. I am just looking for a rough guide what kind of software I should look into and which step follows which step..
As geoserver is fully compliant to WFS, including the "Transactional WFS" level of service. You can provide a running instance with GML to write to its storage with any WFS-T client. More info about this capability of geoserver is available here:
If you need an open source WFS-T ETL tool, you can use the current daily builds of HALE 3.0.0 (the release version of 2.9.0 doesn't yet have this capability):
I good tutorial on how to set up a wfs in GeoServer can be found here: http://www.gistutor.com/geoserver/11-beginner-geoserver-tutorials/36-setting-up-a-wfs-in-geoserver-using-a-shapefile.html
The steps are for a shapefile. However I assume they are very similiar for a GML File.