  The metal mass fractions of gas giants are a powerful tool to constrain their
formation mechanisms and evolution. The metal content is inferred by comparing
mass and radius measurements with interior structure and evolution models. In
the midst of the JWST, CHEOPS, TESS, and the forthcoming PLATO era, we are at
the brink of obtaining unprecedented precision in radius, age and atmospheric
metallicity measurements. To prepare for this wealth of data, we present the
GAS gianT modeL for Interiors (GASTLI), an easy-to-use, publicly available
Python package. The code is optimized to rapidly calculate mass-radius
relations, and radius and luminosity thermal evolution curves for a variety of
envelope compositions and core mass fractions. Its applicability spans planets
with masses $17 \ M_{\oplus} < M < 6 \ M_{Jup}$, and equilibrium temperatures
$T_{eq} < 1000$ K. The interior model is stratified in a core composed of water
and rock, and an envelope constituted by H/He and metals (water). The interior
is coupled to a grid of self-consistent, cloud-free atmospheric models to
determine the atmospheric and boundary interior temperature, as well as the
contribution of the atmosphere to the total radius. We successfully validate
GASTLI by comparing it to previous work and data of the Solar System's gas
giants and Neptune. We also test GASTLI on the Neptune-mass exoplanet HAT-P-26
b, finding a bulk metal mass fraction between 0.60-0.78 and a core mass of
8.5-14.4 $M_{\oplus}$. Finally, we explore the impact of different equations of
state and assumptions, such as C/O ratio and transit pressure, in the
estimation of bulk metal mass fraction. These differences between interior
models entail a change in radius of up to 2.5% for Jupiter-mass planets, but
more than 10\% for Neptune-mass. These are equivalent to variations in core
mass fraction of 0.07, or 0.10 in envelope metal mass fraction.
