Gulfstream G200 (IAI 1126 Galaxy)
Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Flight Crew Last 90 Days 90; Flight Crew Total 9000; Flight Crew Type 400
Training / Qualification; Confusion
Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Ramp
Flight Crew Regained Aircraft Control; Flight Crew Took Evasive Action
Aircraft; Human Factors
After clearing customs I sent my first officer to FBO and I went to Aircraft X and a heavy rain started. [I] called my copilot and told him to stay at the FBO and I'll reposition the aircraft alone. I accomplished the APU and left engine start flow followed by the after start check list. Then; I called Ground for permission to taxi from customs to FBO. After being cleared to taxi; I decided to taxi the aircraft with only the LH engine; when I gave power to the aircraft the aircraft started moving and I had no steering and no brakes. After applying the emergency break and shutting down the engine; the aircraft ended up on green grass area a few feet away from the US Customs parking area. The aircraft was not on the taxi way safe area and no damage to property or Aircraft X occurred.  Airport personal showed up and a tow service was requested to take the aircraft out of the grass zone.
I called and made contact with Pilot (P1). I asked P1 if the G200 aircraft would have steering or normal brakes with the number one engine started. P1 stated the G200 would not have steering or normal braking with only the number one engine running. P1 was in a hurry; in a confined area and without his other pilot. P1 wanted to use only one engine to have less jet wash on the ramp.
G200 Captain reported he taxied single engine without copilot.  Aircraft did not have steering and brake capability and emergency breaking was used to shut down engine; came to stop in grass.