If we have not a program under execution we have the ability to load a file from disk. Use file open button near «Filename» box, or from file menu.
LaserGRBL support G-Code with .nc and .gcode extension. If your file has a different extension but they are G-Code, you can rename your file to use this extensions.
LaserGRBL also support opening pictures and raster images in bmp, jpg, png and gif file format.