Different things work for different people. 
In my case, for example, I am quadriparetic and have great difficulty opening the door on my own, even just to go out. I want to be able to unlock my doors by voice, but I have taken various safety considerations into account, including testing whether shouting from outside the house would work (it doesn’t). So I unlock the door by voice and then my service dog physically opens the door. That way he can’t let himself out if I am asleep, but I don’t have to touch the door.
In the OP’s case, there are several different ways to make this work. You can just use IFTTT with Google assistant for the “if” and smartthings for the “that.”
Or You can set up a routine that unlocks the lock and then have that routine trigger from a virtual switch and then have Google turn on the virtual switch.
Or there are some other things that you can do as well but I think those two are the simplest.