THIS!
Gerbils are like crack for Ball Pythons. Once they get going on gerbils it can be very very hard to get them to switch back to rats. There were even a few stories floating around a couple years back about a BP that, in effect, starved to death because it wouldn't switch back to rats when the owner's gerbil supply dried up.
You asked about feeding gerbils all the time. You must be breeding your own or run a gerbil rescue, because that doesn't sound cost effective to me.