If a gear is used on the shaft of the motor, then assuming:
- $T_M$ is the motor torque (also the gear input)
- $n_M$ is the rpm (also the gear input)
- $T_g$ is the gear output torque
- $n_g$ is the gear output rpm
- $i$ is the gear ratio (assume that $i>1$ means a reduction of rpm)
then the following equation hold:
$$T_g = i\cdot T_M, \qquad n_g = \frac{n_M}{i}$$
So there is a tradeoff.
Torque (a bit more complex)
Torque of a stepper motor is a bit more complex. I.e. the relationship between torque and rpm is not linear, but in reality is more similar to the image below.

So, with increasing rpm on the motor the Torque output is affected. So you might need to consult the datasheet of your motor manufacturer.
Additionally, other factors like the selected driver of the stepper motor can have an effect depending on the supplied voltage.