In Gulfstream Flight Test average pay is about a buck and a quarter and tops out at around $155k. Expect to work five days a week and one weekend a month.
Demo gets less. The department has recently changed their hiring practices bringing new hires in through the AirBorne Product Support G100 and progressing them through the Israeli Aircraft Industries aircraft to the Gulfstreams on a merit basis. In Demo, you get one day off for every day that you are gone minus one day. If you have nothing to do you are not expected to do it at work. Expect to start just above $70K. Demo compensation maxes out at about $154K for those in leadership roles.
Gulfstream offers a defined benefit pension plan, 401K plan with company match, an incentive award program and an excellent health care plan supplemented with an on premises health clinic.
In practice military pilots are preferred for both departments although experience and professionalism can offset that requirement. Leadership is Air Force, Marine Corps and Navy test pilots with an Army TP ascendant.
The Gulfstream Flight Department is a great place to gain knowledge and experience about aviation from a rather august body of pilots which includes a former Presidential pilot, a Vice Presidential pilot, the Chief Test Pilot B-1 Bomber, the Chief Test Pilot B-2 Bomber, the former Chief Pilot Atlas Airlines, Test Pilots who have taught at both Edwards TPS and the Naval Test Pilot School at Paxtuxent River, an Edwards Experimental Test Pilot who is also an aeronautical engineer, an A&P and an Inspector (there's an aeronautical engineer in Demo, too); a Space Shuttle Simulator pilot and one guy who has 542 hours in the Space Shuttle and two landings (he is given grief because he never got current). The Chief Test pilot is also a PPE which helps him to rate his pilots.
Demo has recently been inundated with applications from JetBlue and Delta pilots.
Good luck!
GV