Any plain text editor will open and edit them - Notepad, Notepad++, vi, whatever
They are XML files, but with a different extension.
They MUST have the same base name as the DDS file they relate to (so for CivSymbolsColor45_DLC_D.dds it MUST be CivSymbolsColor45_DLC_D.ggxml) and I'm pretty sure case is significant.
The width and height attributes in the <texture> element must match the dimensions of the DDS file
The <styles>, <import> and <tools> elements can all be "minimal" (not sure if you can completely omit them, but you can certainly remove just about everything from them)
The following is the ggxml file for my Pin Fonts (four 22x22 icons per row, two rows)
Code:
<?xml version="1.0" encoding="UTF-8" ?>
<glyphgen>
<textures width="88" height="44">
<texture name="" src="PinFont.dds" allowcolor="1" alloweffects="1" inuse="1" />
</textures>
<styles count="1"><style name=""><layers count="1"><layer tex="" /></layers></style></styles>
<glyphs count="8" fudgeadv="0" ascadj="-6" spacing="0" height="0" ascent="0" descent="0">
<glyph ch="1" u="0" v="0" width="22" height="22" a="0" b="22" c="0" originx="0" originy="22" />
<glyph ch="2" u="22" v="0" width="22" height="22" a="0" b="22" c="0" originx="0" originy="22" />
<glyph ch="3" u="44" v="0" width="22" height="22" a="0" b="22" c="0" originx="0" originy="22" />
<glyph ch="4" u="66" v="0" width="22" height="22" a="0" b="22" c="0" originx="0" originy="22" />
<glyph ch="5" u="0" v="22" width="22" height="22" a="0" b="22" c="0" originx="0" originy="22" />
<glyph ch="6" u="22" v="22" width="22" height="22" a="0" b="22" c="0" originx="0" originy="22" />
<glyph ch="7" u="44" v="22" width="22" height="22" a="0" b="22" c="0" originx="0" originy="22" />
<glyph ch="8" u="66" v="22" width="22" height="22" a="0" b="22" c="0" originx="0" originy="22" />
</glyphs>
<imports count="0"/>
<toolonly>
<metainfo pt="0.000000" width="256" height="256" alphabias="64" maxsize="256" bestfit="0" pow2="1" freetype="2" fontsrc="" />
<glyphsets><set name="GlyphSetAscii" /></glyphsets>
</toolonly>
</glyphgen>
HTH
W