load from stdin and save to stdout
There are several problems when writing too large gzip files.
Usually it is better to split data among several gziped files.
This would be a lot easier if graph-tool supported receiving the graph from stdin and writing it to stdout, so the "slicing/dicing" job could be left to a complementary application.
i.e.
$zcat graph_*.gz | graph-tool --load=- --save=- | zslicer -m 100 --prefix graph
zslicer, in this case, is a program taking the stream and saving every 100MB in a sequence of gziped files