The g18 file extension is sort of MIDI sequence format file. This format is produced from some sequencers released from Come-On Music's products, Recomposer series ( These are only famous in Japan). Since these format is very popular in Japan, many MIDI sequence data based on these format instead of Standard MIDI file (SMF) are existing. There are even some free software sequencers those are capable handle these format.
How to open:
This file type is not meant to be opened directly, there is no software that could open and work with it directly, or there is no information available in public sources about opening this file type. This is usually the case of some internal data files, caches, temporary files etc.
How to convert:
As far as we know, this .g18 file type can't be converted to any other file format. This is usually the case of system, configuration, temporary, or data files containing data exclusive to only one software and used for its own purposes. Also some proprietary or closed file formats cannot be converted to more common file types in order to protect the intellectual property of the developer, which is for example the case of some DRM-protected multimedia files.