gam·ma-bu·tyr·o·lac·tone (g?m??-byo?o?t?-r?-l?k?t?n?)n. Abbr. GBL A chemical that has many industrial uses, especially as a solvent. It is a chemical precursor to gamma-hydroxybutyrate and is ingested illegally for its euphoriant and sedative effects.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.