Emacs will work fine, especially in nxml-mode if you have the relevant schema installed. But what nxml-mode lacks is the proper XML author controls which the older psgml-mode had: unfortunately psgml-mode won't work with schemas, only DTDs. SGML-mode will be useless for GML. There is also a good xslIDE-mode if you need to work on XSLT for transforming GML to other formats.
Real-time rendering isn't possible in Emacs AFAIK: you would need a separate rendering engine, and I'm not familiar with them. However, you could look at the work that has been done for LaTeX in AUCTeX, passivetex, activetex, and other near-real-time rendering of graphical output: perhaps there are ways to adapt the principle to using a GML rendering engine.
Beyond Emacs, there is nothing available free for editing XML at this scale. oXygen is an excellent general-purpose XML editor, and so are the dozen or so others, but I am not aware of any that have adaptations for GML (but it's not my field, so don't believe me).