GLA is categorized as an n−6 (also called ω−6 or omega-6) fatty acid, meaning that the position of the first double bond in fatty acid counting from the methyl end i.e. an omega-6 fatty acid's first double bond occurs six carbons from the methyl end of the molecule.
In the physiological literature, GLA is designated as 18:3 (n−6). GLA is a carboxylic acid with an 18-carbon chain and three cis double bonds.
It is an isomer of α-linolenic acid, which is a polyunsaturated n−3 (omega-3) fatty acid, found in rapeseed (canola), soybeans, walnuts, flaxseed (linseed oil), perilla, chia, and hempseed. GLA is obtained from vegetable oils such as evening primrose (Oenothera biennis) oil, blackcurrant seed oil, borage oil, and hemp seed oil. GLA is also found in edible hemp seeds, oats, barley and spirulina. Each contains varying amounts of the fatty acid, vary in whether the GLA is found as a free fatty acid or as part of a triglyceride and also vary in position on the triglyceride chain. Conventional borage oil has a GLA content ranging from 15% to 20% while evening primrose oil ranges from 8% to 10% GLA.