Yes, if the Optimum Load Impedance is as shown there, the corresponding-and has to be presented to input- Input Impedance has been given by ADS.You have to walk around these impedances to get the satisfying result.
If this amplifier is unstable under 1GHz, it's a problem because its' harmonics will disturb the operating frequency band.There are many techniques to make any amplifier stable.
But first of all, you should check this unstability continues when the amplifier is terminated with mentioned impedances ?? The stability may chamge much with terminating impedances ( source and load ) therefore the stability must be checked first with those impedances.If the status is persist you can apply stabilization techniques to the amplifier.