Having been in a truck with a lift gate the past 4 years I cant say it is a wise investment. More of a feel good customer service option.
A couple more loads a year , yes.
Things to factor in: additional weight added to the truck. If you are a single axle D unit, the extra weight may make you a C unit. Now you would need a tag axle to be a D unit again.
Also, there will be a fuel loss ( mpg ) by adding a liftgate. From both weight addition and added drag. That coupled with the cost might push the return out a few years.
It has helped, as the ATeam said, providing a little maneuvering room, on a fully loaded truck.
Will you gross an additional 20,000 / year, I doubt it. 5 to 10,000, quite possible.
The run you take to utilize the lift gate may pay better than a non lift gate run you could have got in its place. The value is the difference not the total run.
Now deduct the expense from having one.
You will have to decide if its worth it.
Of course if you want to become WG, whether it ever gets used at all, it is required.