dot is part of the graphviz package, right, in Ubuntu at least? It has Windows binaries, as well, so win users can use the dot executable.

dot -Tgml -o output.gml input.gv

for making .gml.

And yEd has win binaries as well.

Thanks! Added this to the mod description.

This dose not seem to work
dot -Tgml -o TechTreeDiagram.gml TechTreeDiagram.gv
Format: "gml" not recognized. Use one of: bmp canon cmap cmapx cmapx_np dot dot_json emf emfplus eps fig gd gd2 gif gv imap imap_np ismap jpe jpeg jpg json json0 kitty kittyz metafile pdf pic plain plain-ext png pov ps ps2 svg svg_inline svgz tif tiff tk vrml vt vt-24bit vt-4up vt-6up vt-8up wbmp webp xdot xdot1.2 xdot1.4 xdot_json