Hesham, you didn't read what I read properly. I said that I had these designations for MANY years in my notes.
The G100, G150 etc. are recent designations, no more than 10 years I would say. Besides, you make a mistake when you hyphenate them — Grumman used hyphens, not Gulfstream (it's G300, NOT G-300).
So even if there is a G550 now, that does not exclude the fact that the Gulfstream II MUST have had a proper G- design number that was from Grumman, not Gulfstream (G-1159 being the out-of-sequence commercial designation).
However, on closer reflection, I think we are BOTH wrong: the "evidence" you provide for the space design being the G-550 seems awfully inconclusive to me; similarly, I believe that Grumman designations worked chronologically, and with the Gulfstream II being a mid-1960s design, there is NO way it could have been the G-550. Probably more something in the G-260/G-290 range. I now wonder if my "550" and "551" could have been a misinterpretation of the engine's designation, which contained "511".
