Graphlet and GTL is old C++ software available as GNU LESSER GPL Free software.

For the people interested in graph algorithms and graph theory this and graphed is still really interesting.

The graphlet program has a graphical user interface implemented using tcl

The re-compilation of graphlet is now called project regraphlet

The maintained and updated source is available on notabug at

 https://notabug.org/mooigraph/regraphlet

The initial version with only the updated GTL graph template library
is also here in regraphlet.tar.gz

This source does include the GML graph data format parser and specification in C

The GML graph format is supported by most Free software projects
for example  ogdf, igraph, tulip, graphviz, gml4gtk

There is also a peg grammar for C, C++ or javascript parsers here in gmlpeg.tar.gz

click here to try the javascript peg parser in your browser

The documentation of GTL and graphlet is excellent and included or take a look here

Here is the documentation of GTL graph template library

Here is the documentation of the graphlet programmers manual

Here is the documentation of the graphscript language

Here is the documentation of the gml graph language

Or as pdf in gml-technical-report.pdf

Because graphlet has a tcl tk gui interface programming help how to update it is at https://wiki.tcl-lang.org/welcome