Ounce (oz/yd²)
The US measure of fabric weight, common for knits and denim.
Ounces per square yard is the imperial equivalent of GSM. One ounce per square yard is about 33.9 gsm, so 180 gsm is roughly 5.3 oz.
US buyers often quote knits and denim in ounces; Europe and Asia usually quote grams. Convert before comparing quotes, and confirm which unit the mill is using.