A gland is a group of cells whose function is to produce and secrete a specific substance (such as hormones, digestive juices, tears, saliva, and milk) either directly into the bloodstream (in the case of endocrine glands) or through a duct or other opening inside or outside the body (in the case of exocrine glands) (Maynard & Downes, 2019).

Glucagon-like peptide -1 is a hormone the gut cells produces in response to food intake. GLP-1 reduces food intake and increases insulin secretion (Gribble & Reimann, 2016).

Glycans are sugar molecules with proteins and lipids in their chemical structure.

The Glycemic Index (GI) is a scale that measures how quickly a carbohydrate-containing food raises blood glucose levels compared to a reference food, reflecting its impact on post-meal blood sugar response.

Glycoproteins are molecules composed of carbohydrates and proteins. Mucin, for example, is a type of glycoprotein that gives the mucus layer a gel-like consistency.

The term “gnotobiotic” refers to the study of organisms inoculated in a carefully controlled environment containing one or a few types of organisms to observe their influence on the body or brain (Williams, 2014). For example, germ-free (GF) animals become gnotobiotic after being inoculated with one or more specific bacterial strains. This method assists researchers in studying the interactions between microbes and hosts.

Gram-staining is a traditional laboratory technique used to rapidly classify bacterial species into two broad categories: gram-positive and gram-negative, according to their type of cell wall.

Gram-negative bacteria contain outer and inner cell membranes and create pink color colonies upon gram-staining. Most of the gram-negative bacterial species are pathogenic due to the presence of lipopolysaccharides (LPS) on their cell membrane. For instance, E.coli is a gram-negative bacteria in our gut and expresses LPS molecules that participate in the MGBA mechanism.

Gram-positive bacteria do not express LPS and have no outer lipid layer except the thick peptidoglycan layer, which gives purple colonies upon gram-staining. Gram-positive bacteria reside in our gut and carry out essential bodily functions. For instance, Lactobacillus and Bifidobacterium are true probiotics.

Gray matter is a type of neural tissue found in the brain and spinal cord that appears grayish in color. It primarily consists of the cell bodies of neurons and their associated dendrites and synapses. Gray matter is crucial in information processing, decision-making, and sensory perception. In contrast, its counterpart, white matter, consists of myelinated nerve fibers that facilitate communication between different parts of the nervous system.