That's strange, my older Canon HJ11 works perfect on the broadcast G2 (except push iris in filmmodus, that's a big disaster)
To let iris work remotely on an Atem you have to set auto iris to 'On' on the lens, go to the iris menu in the G2 and set auto iris to 'On' also. After that set auto iris on the lens to 'Off'.
It's a strange workflow but unfortunately there are more 'strange behaviors' with Blackmagic equipment