Gulfstream G100/G150 (IAI 1125 Astra)
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 150; Flight Crew Total 6800; Flight Crew Type 2500
Situational Awareness; Workload
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Human Factors; Procedure
Established on the GIBBZ2 RNAV arrival at 6;000 feet approaching GIBBZ we were advised to expect the ILS 1C approach.  Initially we were expecting the ILS 1R so we had to reset the FMS to the ILS 1C approach.  Resetting the FMS also enables the Localizer to auto-tune the correct frequency and set the inbound course.  After resetting for the ILS 1C we were just crossing the GIBBZ intersection. The FMS was updating the route with the new ILS when were just about to make the turn to continue to follow the arrival for landing North at IAD.  In the process of this FMS updating the LNAV (Lateral Navigation) mode of the flight director and autopilot disconnected.  The aircraft then  proceeded straight ahead and did not make the turn to follow the STAR south as is required for landing North at IAD.  During this time the cockpit was busy as I was re-briefing the approach for the ILS 1C and confirming the localizer frequency and minimums that were set. We did not notice that the aircraft left the LNAV course and had not made the turn.  ATC advised us that we had missed the turn at GIBBZ and gave us a heading to follow.  They then advised us to expect the ILS 1R.  Now on vectors we reset the FMS again for the ILS 1R and re-briefed the approach.  The approach and landing were normal. As the Safety Officer for the company I will review this at the next safety meeting.  I feel that if the ILS assigned is changed it greatly increases workload for the crew when close to the airport.  It would help in future if runway or ILS assignments were provided well in advance.  It would also help if they were not changed after they have been assigned because the crew has already set up for that approach and briefed it.  I will ensure all our crews will better monitor the RNAV STARs and also keep a sharp watch of the FMS and autopilot modes during changes to STARs or approaches.
[Report narrative contained no additional information].
G150 flight crew reported they deviated from cleared track on arrival into IAD; late runway changes were cited as contributory.