GC

The GC gene (known as the group-specific component gene) is part of the albumin gene family and encodes the vitamin D binding protein (DBP), which binds vitamin D and transports it to its target cells and tissues. 

It is better to have this gene increased or upregulated most of the time. 

The GG genotype is associated with lower 25(OH)D concentrations.  Improving your vitamin D levels include increasing your dietary vitamin D intake, UV exposure, and supplementation.

The TT genotype is associated with lower D binding protein (DBP) levels and lower serum vitamin D levels. The T allele may also be associated with an increased risk for the development of metabolic syndrome, COPD, and certain cancers, especially when vitamin D levels are insufficient. Improving your vitamin D levels include increasing your dietary vitamin D intake, UV exposure, and supplementation. 

When looking at your overall need for vitamin D, it is important to consider your genotype for VDR, DBP, and CYP2R1 too.