Available from Flight Safety International for about $54k.... and they throw an Initial Type Rating in for free.
You could also have Gulfstream Planebook subscriptions set you up for the bargain price of $1k (that’s an annual fee btw). Though you do need the sponsorship of a G5 or G6 operator for that.
You may find them online, though I wouldn’t count on it. The reason you can’t find a Gulfstream type on any MS Flight Sim is because Gulfstream want it that way. I’m sure someone will correct me on that...
Now if it were a Dassault airplane, I could email you the lot.