From Wikipedia, the free encyclopedia. Grapefruit seed extract (GSE), also known as citrus seed extract, is a liquid extract derived from the seeds, pulp, and white membranes of grapefruit.Self-made natural GSE processed in the laboratory without synthetic agents is prepared by grinding the grapefruit seed and juiceless pulp, then mixing with glycerin.ommercially available GSE sold to consumers are made from the seed, pulp, glycerin (and in some cases synthetic preservatives) all blended together. Grapefruit seed extract is sold as a food supplement and used in cosmetics because it is a claimed natural antimicrobial.