Do you know any good tutorials or any resources for learning from the beginning GML?
I am asking because (later) i would like to learn how to make a application schemas in context of INSPIRE.
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I'd suggest starting from http://www.w3.org/Mobile/posdep/GMLIntroduction.html.
If you're planning to work with application schemes in INSPIRE, you'll also need a thorough understanding of XML and XSLT (Extensible Stylesheet Language Transformations).
I think the task doesn't require in-depth understanding of UML. You'll have to be able to read it. Anyway, you can find tons of resources at http://www.uml.org/.
You may be able to glean some general info about INSPIRE from some of these links: