Upload & Sell: Off
The ppi (pixels per inch) is meaningless for images viewed on computers and nearly meaningless for modern printers, so ignore it. Just crop your pictures to a 192 to 120 ratio or 96 to 60 ratio or a 48 to 30 ratio or a 16 to 10 ratio or an 8 to 5 ratio if you prefer. See how it is simple arithmetic?
Photoshop Rectangular Marquee or crop tool gives you the choice to set for a fixed ratio. Use it. It think GIMP is the same.
Once cropped to the right ratio, then resize to 1920 x 1200.