Gtg, GTG, gtg, or G2G may refer to:
- Grantsburg Municipal Airport (IATA code)
- Gauge theory gravity, a theory of gravitation
- Geophysical Tomography Group, of the Institut de Physique du Globe de Paris
- G2G Racing, an American stock car racing team
- GTG, a codon in the DNA codon table for the amino acid valine
- G2G, government to government, in e-governance