Recently we were thinking of getting gerbils, and we already have a tortoise. We were wondering if the tortoise would mind the gerbils if they were both out of their cages at the same time, or vice versa. Would the gerbil be intimidated by the tortoise?
Gerbils are usually not aggressive and don't tend to bite without a reason like being stressed or attacked. I assume you have a terrestrial tortoise and these are typically herbivorous, thus not interested in attacking nor eating other animals. However, one can't really guarantee that a tortoise won't misinterpret gebril's curiosity as a threat or for whatever reason snap it by mistake. Even herbivorous tortoise's jaws are quite strong and capable of doing severe damage to such small animals as gerbils. It also depends how do they compare in size.
Also, gerbils as all rodents need to constantly wear down their teeth by biting hard materials and one should be careful that the tortoise's shell won't attract such attempts.
So I think in conclusion it should work out, especially if the tortoise is not much larger, these spiecies aren't natural enemies. But at the beginning it absolutely needs to have your constant supervision. After some time they should just get used to and ignore each other, which would be the optimal outcome.