I measured a 2 different pHEMT amplifiers from 2 separate companies. I used GSG probes for the input and output, and DC probes for biasing the drain and gate.
When I do this, the gain is signifantly lower than from the spec sheet.
When I add a external 1k series resistor between the DC supply and the DC probe at the gate, I get the advertised gain.
Why? Was I seeing oscillation? Is it because the gate needs current limiting?
Do you known the concept of Bias Tee?
You should insure that DC bias port is very-high impedance (open) for RF frequency.
The high resistance series resistor you used paved the way.
Your circuit is not a simple GaAs pHEMT instead an amplifer with its own biasing.
Gate biasing is negative to Source in pHEMTs and it's provided by simple source resistance so it creates a negative GS voltage when G is grounded.
I think you have forgotten to connect RF choke btween Vdd and the circuit because some circuits don't have own on-chip RF chokes, this is connected externally.