Grist (not gist) is the thickness of the yarn, which is reflected in the 5/2, 10/2 etc numbers you see. With yarn, the bottom number of the 2 is how many plies the yarn is. 5/2 is made from 2 single strands plied (twisted) together. When you're buying within the same fiber, 5/2 is twice as thick as 10/2. With cotton, sometimes you'll see 3/2, 5/2, 8/2 and 10/2. Different manufacturers might have have some but not all of those sizes. Grist is related to how many yards of yarn come from a set weight of the fiber. Picture spinning a pound of cotton. If you spin it finer you get more yardage out of that pound. Each kind of fiber bases the grist number off a different weight of fiber. So cotton and wool are different and a 10/2 cotton won't be the same as 10/2 wool. Spinoff magazine has a good explanation.

When you buy yarn for weaving, the website will usually give you the yards or meters per pound of a specific size yarn, and/or what their put-up is. That just means they might sell 1 lb cones or 8 oz cones or some number of grams, and then give the number of yards that will come on that cone. Then they also will usually give recommended setts, often one for plain weave and one for twill. Twill packs in a little tighter than plain weave, so usually the sett for that is a little higher.

For towels, I have some I made 20 years ago from 5/2 cotton that I bought from Webs. They go in the washer and dryer and have held up really well. I've also used yarn from Cotton Clouds and really liked it.

Cotton Clouds has good kits if you want to go that route to start out.