I don't know what a gfg block is. But, for the most part you need to be mindful of the quarter inch seam that you must have on each edge for the block to set in properly. I use square up rulers to square mine up. I use the one that is closest to the size I am squaring so it is easier not to make a mistake with the right squaring size.
I place the diagonal line down the center from tip to tip (checking the numbers for the unfinished block size) you will look through the ruler and be sure to see that there is a full quarter inch above the seam allowance line at the top and on the cutting side. Then cut that side and the top, turn and do the same to finish.
Last edited by RedGarnet222; 01-15-2013 at 11:20 AM.