The answer depends on which Gaussian pulse is in use. There is a simple gaussian (for which I give hopefully the real answer below), there are also G3 (consists of three different gaussian pulses back-to back and is used for inversion) and G4 (four-pulse cascade used for the excitation) pulses. Cascade pulses have excitation or inversion profiles closer to rectangular.
Gaussian pulse can also be used for selective saturation (2,4) (which is applied for detection of binding of small molecules to large bio-molecules in STD experiments) , where a train of pulses separated by a short delay is applied for some period of time. Here selectivity will depend on the total duration of saturation time and parameters of the pulse, but it should be easy to run an STD-type of experiment with a small molecule alone first to make sure that saturation does not affect it.
Setting up single gaussian pulse
Here is the link to the original paper for a single gaussian pulse. A 10ms gaussian pulse calibrated for 90 degree on-resonance flip angle will introduce almost no excitation at offsets >300 Hz.
So if you apply a single gaussian pulse like that for selective excitation the closest other resonance must be farther than 300 Hz away. If you need better selectivity - then just proportinately increase duration of the pulse and re-calibrate it again so that it has 90 degree flip.