Giteau will join French giants Toulon following the World Cup on a two-year deal which runs until 2013.
But the Brumbies captain could yet play against the Lions if he returns home following his stint at Toulon.
Giteau outshone the returning Rebels and England fly-half Danny Cipriani in the triumph.
Giteau provided the scoring passes to Christian Lealiifano, Tyrone Smith and Pat McCabe, while Stephen Moore also crossed for a try.
"I think it's just nice for the group ... achieving something we can be proud of going into the back end of the season," said Giteau.
"We didn't just want that Reds win, we wanted this one as well. We wanted to finish the season on a high."