CE560 FLT CREW HAS A RWY INCURSION AT GCK AFTER RECEIVING AND NOT QUESTIONING UNCLEAR TAXI INSTRUCTIONS.

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WE USED GCK FOR A FUEL STOP AND AFTER ENG START AT THE FBO; WE CALLED GND FOR TAXI. GND SAID 'TAXI RWY 35 CHARLIE.' THE SIC SAID TO ME THAT WASN'T SO CLR. I HAD STARTED TO TAXI AND TURNED TO THE L AND COULD SEE TXWY C AND A TWIN CESSNA TURNING S ON RWY 35. SINCE THE TAXI WAS VERY SHORT TO TXWY C AND THE PARALLEL TXWY WAS PARTIALLY CLOSED TO THE S; I ASSUMED THAT WE WERE GOING TO BACK-TAXI BEHIND THE TWIN CESSNA AND EXIT THE RWY AGAIN S OF THE CLOSED AREA. WE WERE RUNNING THE CHKLIST AND I LOOKED UP FROM SETTING THE TKOF HEADING AND THE TWIN HAD MADE A 180 DEG HDG ON THE RWY ABOUT 200 FT S OF THE TXWY C I STARTED BRINGING OUR ACFT TO A STOP AT THE SAME TIME GND TOLD US TO HOLD OUR POS. WE STOPPED JUST PARTIALLY OVER THE HOLD SHORT LINE. I THEN HEARD THE GND CTLR CANCEL A TKOF CLRNC FOR THE TWIN CESSNA. THE CTLR WAS WORKING BOTH GND AND TWR. WE HAD NOT HEARD THE TKOF CLRNC ISSUED TO THE OTHER ACFT AS WE WERE RUNNING THE CHKLIST. GND ASKED US TO MAKE A 180 DEG TURN ON TXWY C TO CLR THE STOP LINE AND THEN HE CLRED THE TWIN FOR TKOF. HE THEN CLRED US TO BACK-TAXI; WHICH WE DID; FULL LENGTH AND ABOUT 1/2 WAY INTO THE BACK-TAXI; THE TWR CLRED US FOR TKOF AND ASKED US TO EXPEDITE OUR BACK-TAXI. WE WERE LURED INTO AN ASSUMPTION AND ON AN ARPT WITH ONLY 1 OPERATIONAL RWY; WE LET OUR GUARD DOWN AND DID NOT CLARIFY WITH GND WHAT EXACTLY WAS INTENDED. IF HE INTENDED FOR THE TWIN TO NOT BACK-TAXI FULL LENGTH HE SHOULD HAVE INCLUDED 'HOLD SHORT' IN HIS TAXI CLRNC TO US. LIKEWISE; WE SHOULD HAVE DELAYED THE CHKLIST WITH SUCH A SHORT TAXI TO THE HOLD SHORT LINE. LESSON LEARNED: NEVER LET YOUR GUARD DOWN AS TO POSSIBLE INCURSIONS!