Hello and welcome! I haven't played with GIMP for a long time, but I think of it as more of a paint/photo program vs. a vector draw and for me it would be difficult to design a patchwork block in it but there may have been some recent enhancements that would lend itself to the task. Although a lot of folks use a computerized program to design their quilts, many, many folks go 'old school' and use a pencil and grid paper. I would start with pencil and paper to design the block. You can figure out fabric requirements for a single block, then use a program to lay out the blocks for your quilt and multiply the number of blocks with the fabric requirements for the single one to determine yardage . This way, the size of grid you use in the application doesn't really matter. I did a google search and it looks like there is at least one board dedicated to GIMP and they have some posts on quilting. you may also want to consider asking there.