That statement doesn't make much sense.
In solidity 0.5 gas is the remaining gas. With sub(gas, 34710) it would seem they want to send all the remaining gas minus 34710 to the called contract, but from the comment it appear they want to send at least 34710.
The error is trying to say that in solidity 0.6 you have to use gas() as it were a function.
I will ...