I can see the reason for bringing Gamble into the side.
MM is unable to play a full 80 at hooker which means that Reynolds will have to cover there for some portion of the game (thankfully meaning the end to ET at 9). With Benji out that means we need a cover for 6 hence Gamble.
Tui could have been an option here but his current form has not been good and this means less position swapping mid game.
If Gamble comes off the bench it also means that we have someone fresh to spark our attack late into the halves. He should also provide a bit more utility cover for the backline than having another hooker on the bench.
On Robbie, I would be ok with him coming back on a limited deal, 1 year max with one more in the clubs favour pretty much like Benji.
He would immediately fix our 80 minute hooker problem and would free up a bench spot for another big forward or impact player. A bench of Aloia, Chee Kam, Huth and Twal would give us a lot of punch up the middle. Means Reynolds can settle into the 6 role and would take some pressure off the halves as we could put on some forward plays off of the ruck. Something we don't get out of any of the current hooker options.
This is all assuming however, that we get a more humble version of Robbie who doesn't go for the hero, ball hogging plays. Big assumption I know.