Gordon does make the most sense to back up but as you say, his goal kicking is needed unless they think Keary is an option.
Between Origin and injuries we will probably be luck to have our best back 5 (Teddy, Toops, Latrell, Manu and Fergo) all available for 50% of games but golden runs with injuries do happen and I would hate to think Latrell or Manu are on the outer.
The problem, at this stage, is that Manu, Mitchell and Ferguson all make bad defensive reads. At this stage, I think Gordon will be in the side to partner either Manu or Ferguson on the right hand side in the centres. He will give his inside partner some stability and experience, as well as trying to get them to not slide in for the big hit all the time, thus leaving gaping holes for the defence to run into.
I think Latrell,barring a massive form slump, is locked into playing at left centre with Tupes outside him.
Besides being experienced, and older than the rest of the back 5, Gordon is the club's no.1 goal kicker. I don't know why you mention Keary as a goal kicker, he's never kicked a goal in the NRL since he broke into first grade in 2013. I think you will find the back up kicker, when fit, will be SST. People seem to forget in 2016 he kicked 27 goals from 35 attempts last year. He used to be a handy kicker when he was in the lower grades.