Where is the description of the format of the data section of saved simulation files?
If you intend to read the simulation files (to convert them or do other operations/calculations etc…) I would highly recommend that you do so through the functions provided by the Gerris library (gfs_simulation_read() and so on). This way you will not reinvent the wheel and you will be able to use all the functionalities provided by the library (traversal of the octree structure, computation of gradients, interpolations etc…). This would also ensure that your code is independent of the format changes in the simulation file.