The NTSB report seems to say that the pilots rushed the takeoff and did not remember that the gust lock was ON thus disabling the elevators. They blame manufacturer for a throttle interlock system that was so ineffective that it allowed the pilots to advance the throttle to 22-26 degrees and thereby reach 187 mph. The wreckage reveals the gust lock lever in the OFF position. I am not a pilot and have never been near a Gulfstream. Here are my questions.
1) How unlikely would it have been for two professional pilots to forget to disengage the gust lock lever. Not likely, you can't get much past idle thrust with it engaged. On the G2/3 with the lock engaged one throttle can be advanced to a point but the other one is limited even further towards idle.
2) Could the plane have reached that "V1" and "rotation" speed with the throttle limited to 26 degrees. Possible, but it would take a lot of runway. Pilots unable to get full takeoff power would have aborted the takeoff immediately.
3) Could the elevators still have been locked with the gust control lock OFF. possible, I have seen the control lock handle stick. If you could not get it unstuck that would be the end of the flight before you even started the engines.
4) Would a pilot routinely test the elevators before reaching V1. yes and no, you test before or as you enter the runway. Testing it just before V1 causes the nose to bounce up and down, rather disconserting to the passengers.
5) How impossible would it have been for someone not on the flight to have tampered with the system. You would have to have access to the airplane and take the throttle quadrant apart.
The control lock is normally disengaged during the before start checklist. See 3 above.
The above answers assume the system is the same as the G2/3. It looks like it is similar, the 2/3 does not have an intermediate setting, I am assuming that with the lock handle partly stowed the throttle could be unlocked but the controls still locked. That would likely require a broken system. It is a system made up largely of cables and pulleys.