GDP-mannose is produced from GTP and mannose-6-phosphate by the enzyme mannose-1-phosphate guanylyltransferase.
The degradation of mannans (and many related forms of hemicellulose) has been well studied.  The hydrolysis of the main mannan backbone is catalyzed by various enzymes including β-mannosidase, β-glucosidase, and β-mannase. The side chains are degraded by esterases and α-galactosidase.
When a long chain of mannan is hydrolyzed into shorter chains, these smaller molecules are known as mannan oligosaccharide (MOS). MOS by definition can be produced from either insoluble galactomannan or soluble glucomannan, although the latter type is more widely marketed.
Glucomannan MOS is used as prebiotics in animal husbandry and nutritional supplements due to its bioactivity.