You can create gradient files (.grd) inside Adobe Photoshop using the native gradient editor, formerly known as Adobe Image Ready. This is likely what jpg to grd conversion represents, making a new gradient from a JPEG picture.
The source can be any bitmap image, including JPEGs or PNGs. Then you have to tweak the gradient and set color stops, opacity and other settings.Open JPG file Open GRD file
One of the best bitmap graphic editing program
Mac version of popular digital image editor
A freeware graphic program