Strength comes from multiple stitches, sharing the load assuming you are not creating stress in the fabric. Lots of stitches X strength per stitch (which, conservatively is the strength of the thread, but in industry calculated at 70% more than that. One row of 3mm stitches (ie 8 stitches/inch) 50 inches long X 3lb/ stitch = 1200lb.....assuming all the stitches bear the weight
BUT: If the thread is weak or weakens
, then it can all fail, starting from the first broken stitches. Be sure your thread rows are locked up and strong at the ends.
Here's one guide to thread size and strength:
Unlike several in HF, I think there is nothing special about Gutterman thread. Thread makers around the world spin as good or better. What matters is picking the right ones and using them correctly.