Possibly damaged. It depends on the phase shift between the FET cell and the open. The worst case is an in-phase reflection back to the drain which maximizes gate-drain voltage. A MOSFET will have holes blown in the gate oxide causing either instant or long term failures. A MESFET can have gate metalization melt, or metal migration in the long term. You can just monitor gate-current on a MESFET but MOSFETs you can't easily tell when you are stressing the device. The best case is an out-of-phase reflection which will just cause the FET to get hotter. You need to know the gate-drain breakdown of your process. It needs to be at minimum 2*VDD but more is better since you can have peaking from the harmonics; class A needs less head-room than higher classes.