Data output file created by Printed Circuit Board (PCB) CAD tools like Altium Designer and used by PCB manufacturing machines to designate electrical connections; contains data for multiple layer boards about the layout for bottom copper layers; similar to the .GTL file extension; written in RS-274X, which is the most common and contemporary Gerber format.
Gerber format files, like GBL, act similar to .PDF files. They're an exported version of the PCB layout and are meant to be viewed and referenced but not edited.
The name of the GBL file is derived from the name of your PCB. Therefore, if your PCB is called "MyProject," the file will be called MyProject.gbl.
NOTE: The Gerber format was originally developed by Gerber Systems but is now maintained by Ucamco.