  Gas-rich galaxy mergers are more easily identified by their disturbed
morphologies than mergers with less gas. Because the typical gas fraction of
galaxy mergers is expected to increase with redshift, the under-counting of low
gas-fraction mergers may bias morphological estimates of the evolution of
galaxy merger rate. To understand the magnitude of this bias, we explore the
effect of gas fraction on the morphologies of a series of simulated disc galaxy
mergers. With the resulting g-band images, we determine how the time-scale for
identifying major and minor galaxy mergers via close projected pairs and
quantitative morphology (the Gini coefficient G, the second-order moment of the
brightest 20% of the light M20, and asymmetry A) depends on baryonic gas
fraction f(gas). Strong asymmetries last significantly longer in high
gas-fraction mergers of all mass ratios, with time-scales ranging from >= 300
Myr for f(gas) ~ 20% to >= 1 Gyr for f(gas) ~ 50%. Therefore the strong
evolution with redshift observed in the fraction of asymmetric galaxies may
reflect evolution in the gas properties of galaxies rather than the global
galaxy merger rate. On the other hand, the time-scale for identifying a galaxy
merger via G-M20 is weakly dependent on gas-fraction (~ 200-400 Myr),
consistent with the weak evolution observed for G-M20 mergers.
