The Gabba gets a lot of dew at night. This often results in "wet weather footy" even if it hasn't rained for days. So, unless there is a deluge, chances are the conditions are going to be the same as would have been the case if it were completely dry.
In one respect, a bit of rain is better for the footballers. The Gabba drains really well (there's a massive stormwater system which runs under the ground) but the Gabba is ordinarily such a flint hard surface that taking a bit of sting out of the ground is good for player welfare.
Lions have been training with this dew for how long? Huge advantage.