  For a general class of gas models ---which includes discrete and continuous
Gibbsian models as well as contour or polymer ensembles--- we determine a
\emph{diluteness condition} that implies: (1) Uniqueness of the infinite-volume
equilibrium measure; (2) stability of this measure under perturbations of
parameters and discretization schemes, and (3) existence of a coupled
perfect-simulation scheme for the infinite-volume measure together with its
perturbations and discretizations. Some of these results have previously been
obtained through methods based on cluster expansions. In contrast, our
treatment is purely probabilistic and its diluteness condition is weaker than
existing convergence conditions for cluster expansions.
