Setting the dpi in the printer driver won't help with getting the correct scaling, nor will it affect anything in the Gimp dialog box. In fact, it's a good idea to put it into draft mode for a test print first to make sure you don't waste a ton of ink on a print that's at the wrong scale. The draft mode in the printer driver will downsample the image prior to printing, essentially reducing its pixel dimensions at the same time as it decreases the dpi. It won't change the scaling unless you tick a box that says something like "fit to page."

Anyway, there's a mismatch somewhere in there. The X and Y Resolution in the window you describe is the dpi, but with the numbers you give your image should be 62.5 inches wide (6250 pixels / 100 px / inch). I'd say the simplest way to get to where you need to be is to count the actual grid squares across, then adjust the resolution until the width in inches is equal to the number of grid squares. Then you'll be certain that one square = one inch.

Don't let it resample anything—you don't want to go throwing away pixels.

My inclination is that you should download Posterazor and let that do your tiling. I have a feeling that putting Excel into the mix might be adding additional needless complexity. It's not really designed for this kind of thing, so it's unlikely that it does it as well as a program dedicated to exactly this job would do.