[graph-tool] Writing a custom binary format (.gt file) in spark

I've got a dataframe inside of Apache spark which I'd like to output as a .gt file. The dataframe is a table of edges between vertices like so: 0 9 0 8 9 1 I read the guide on the custom gt file format, but how do I actually write one for my edges dataframe? That too, inside of spark? Any ideas/examples you could point me to? Any help is appreciated, thank you!