  This paper describes the first catalogue of photometrically-derived stellar
mass estimates for intermediate-redshift (z < 0.65) galaxies in the Galaxy And
Mass Assembly (GAMA) spectroscopic redshift survey. These masses, as well as
the full set of ancillary stellar population parameters, will be made public as
part of GAMA data release 2. Although the GAMA database does include NIR
photometry, we show that the quality of our stellar population synthesis fits
is significantly poorer when these NIR data are included. Further, for a large
fraction of galaxies, the stellar population parameters inferred from the
optical-plus-NIR photometry are formally inconsistent with those inferred from
the optical data alone. This may indicate problems in our stellar population
library, or NIR data issues, or both; these issues will be addressed for future
versions of the catalogue. For now, we have chosen to base our stellar mass
estimates on optical photometry only. In light of our decision to ignore the
available NIR data, we examine how well stellar mass can be constrained based
on optical data alone. We use generic properties of stellar population
synthesis models to demonstrate that restframe colour alone is in principle a
very good estimator of stellar mass-to-light ratio, M*/Li. Further, we use the
observed relation between restframe (g-i) and M*/Li for real GAMA galaxies to
argue that, modulo uncertainties in the stellar evolution models themselves,
(g-i) colour can in practice be used to estimate M*/Li to an accuracy of < ~0.1
dex. This 'empirically calibrated' (g-i)-M*/Li relation offers a simple and
transparent means for estimating galaxies' stellar masses based on minimal
data, and so provides a solid basis for other surveys to compare their results
to z < ~0.4 measurements from GAMA.
