Ok, so from looking at the second picture, your board also appears to have the proprietary front panel USB ribbon cable. I have no idea what form factor that is. It looks like it has 5 expansion slots. Too wide to be mATX, but not wide enough to be regular ATX. Hard to tell from the angle where the standoff holes are.
Also, what were you planning on doing about the CPU cooler? That Dell motherboard doesn't have the standard cooler holes, so you'd be stuck with the Dell heatsink. That, in itself isn't bad since it's a heatpipe cooler. But there's also that big green plastic shroud that the Delta fan is attached to, and it's clipped onto the backplane of the Dell case, not onto the heatsink itself. You probably wouldn't be able to use it on the Gigabyte case since it's attached with specialized plastic clips, not screws.
In short, it seems like there are quite a few little potential issues with your plan.