Why does my bandpass RF filter complain about Ts in Simulink?
if I set it lower, then I would have aliasing issues.


You said you have a carrier of 2 GHz and a sampling period of 2.5 ns.

You already have aliasing issues.

You need a sampling period of less than 0.25 ns to avoid aliasing.

It may be that you are deliberately undersampling in order to use the sampling process to down-mix your signal. That is a perfectly acceptable technique, although I'm not familiar enough with Simulaink to know if there's any isses with simulating it.

On top of that, if a "Gaussian Noise Generator" is in fact a white gaussian noise generator, then you have an infinite signal bandwidth. (I'm not clear if you are using this as a stand-in for a message signal or to model channel noise)

Try low-pass filtering the noise source down to a small fraction of the carrier frequency before using it to modulate the carrier. If you keep your 400 MSa/s sampling rate, you need to keep the signal bandwidth below 200 MHz to avoid aliasing.