How do I open and read a GeckoBootLoader file?
.gbl files are binary files created by calls into Simplicity Commander and they are not editable files. From the command line Simplicity Commander does have a 'gbl parse' command, I am not sure if that is what you want. Please see "UG162: Simplicity Commander Reference Guide" for more details. This is the relevant section of that manual.
6.7.8 GBL File Parsing
Parses a Gecko Bootloader (GBL) file and writes the application image to the specified filename. Optionally decrypts an encrypted GBL
file. The keyfile must be the same as was used for encrypting the encrypted GBL file.
Command Line Syntax
$ commander gbl parse <gbl filename> --app < filename> [--decrypt <key filename>]
Command Line Input Example
$ commander gbl parse nodetest.gbl.encrypted --app app.s37 --decrypt key.txt
Command Line Output Example
Reading GBL data... Decrypting GBL... Reading application... Writing application to app.s37... DONE
Also your post got uploaded twice which happens sometimes, I will go ahead and delete the duplicate post.