I would venture that if this person concerned was looking to have a go in his G5. Then its probably a very wealthy owner that may just want to go do the course and understand a little more about the $50 mil investment he's just made.
Cant see a problem with him in the RHS doing the odd trip in summer from cavok apt to cavok apt with some crusty capt in the LHS politely keeping him between the rails. I would iamgine the "proper" FO would be on the jumpseat.
If my boss took the time and trouble to take time from his very busy day and manage to get a TR (as somebody said before they don't give any gulfstream rating away in cereal packets regardless of your perosnal balance sheet) then good luck to him.
It keeps his interst in aviation, lets him brag about flying it himself at dinner parties. Decidedly uncomfortable for all the pro's involved I grant you but these guys are incredibly successful and dont have a lot of boxes to tick left!
now if its some wannabee C152 pilot asking the question then the answer is NO!
Bluevolta - it can be done on a PPL, you cannot act as P1 in easa without a valid ATPL
Happyjack - its only normally declared captains nowadays with a stipulation in private ops that all crew have attended sim training in the last 24 months
Happy to be proved wrong etc etc