If you look closely at the scene where Hela is next to the (fake) infinity gauntlet in Odin's vault, you'll see that the gauntlet is right handed.
I'd say that there really isn't anything significant about the gauntlet being left or right handed, Tony was right handed and so it was probably a natural decision for him to put the stones on the right hand of his armour.
It's most likely down to personal preference, as A J♦ suggested, maybe Thanos really was left handed and preferred to have the gauntlet on his left hand.
Maybe the dwarves made the gauntlet left handed because that was the mould they had, or it could have been the only mould left for the gauntlet after they made Odin's right handed gauntlet.
Either way, Tony, Bruce and Thanos were the most powerful beings in the universe when they had the glove, I highly doubt they were paying attention to what hand it was on.