Hi,
Yes, your amplifier will oscillate at those freuencies where |S_11| and/or |S_22| are >1.
If you run a transient analysis by first removing the "PSIN" and not attaching any "VSIN", you will notice that there is a finite multi-harmonic sinusoidal signal present on your output.
This multi-harmonic sinusoidal output will be the superposition of sinusoidal signals whose frequency will be in the range where |S_11| and/or |S_22| are >1.
In simple terms, it will be a sum of sine waves from DC to 6GHz!!!
Naren