glm = grammes per linear metre
gsm = grammes per square metre

roughly speaking what we think of 16 oz cloth is about 500 grams per linear metre

unless you actually know the threadcounts of each tartan, the epi (ends per inch = number of threads) isn't going to be the easiest way of working out the sett size, as it will still need some calculation.
It would be so much easier if it was just documented, even if it wasn't absolutely exact, an idea of whether it was 6" or 6.5" would be a great help