[in menu Analyse]
This batch operation checks all MIDI files for being valid in specified MIDI folder. At completion a list of errors is shown and you are asked if you want that GNMIDI tries to repair the files. Repaired files will be opened in a document window. You need to check if the repairing is acceptable and then save the MIDI file.
Checking same folder again will work quicklier, since it won't check those files again which didn't change. If you check all files in this folder again then you can delete the file with extension .chk