Saracens also had a player on his way back from injury, with Richard Hill completing the first 40 minutes on his come-back from injury.
Saracens came out the blocks the stronger with a try from Morgan Williams within three minutes, followed by a Tom Cox (second row) try on the half hour.
Gloucester gradually got into their stride with McRae hitting some good kicks and putting the exciting back-line into space.
The forwards started to dominate, as Saracens hardly got a look-in in terms of possession and territory as the first half progressed.
Gloucester hit back with a converted Jack Adams try before half time, when McRae's pass split the Saracens defence inside their own 22 and the St Peter's School pupil went over.
James Simpson-Daniel was on top form in terms of defence, with some crunching tackles, Terry Sigley was prominent in the loose, and Simon Amor's effort showed how much the team wanted to impress and push for a first team slot.
Noel Curnier came on at half time, and the Kingsholm club went on to score 29 unanswered points in the second period.
Mark Foster, playing at inside centre, bagged the first score of the second half, before a superb run from trialist Simon Hunt (currently with Pertemp Bees) set up Curnier for his first try. Andy Gomarsall's superb touchline conversion took Gloucester into a 19-10 lead.
The French hooker scored his second try of the evening six minutes later on 72 minutes, before St Peter's School pupil Vince Lanciano touched down, followed by captain Luke Narraway's dive for the line in injury time.
Aussie stand-off McRae played 65 minutes, and received a superb reception as he was replaced.
The mix of experience and youth paid off for Gloucester as they dominated the game, and the watching Nigel Melville and Dean Ryan must have been impressed with the number of talented youngsters coming through the ranks.
15. Olly Morgan, 14. Simon Amor, 13. James Simpson-Daniel (Simon Hunt 57), 12. Mark Foster (Jack Adams 30-40, Vince Lanciano 78), 11. James Bailey, 10. Duncan McRae (Dean Moahy 62), 9. Andy Gomarsall (Nick Runciman 80), 1. Terry Sigley, Rob Elloway (Noel Curnier 40), 3. Emyr Lewis (Paul Price 40), 4. Jonathan Pendlebury (George Evans 80), 5. Dan Tuohy, 6. Will Matthews, 7. Will Jones (Dean Miller 39)