Since drag equals thrust in level flight, we can work out an approximation for any aircraft that has a counterpart in MS Flight Simulator or similar software.
Before you fly away, real values would require renting a CFD / wind tunnel facility, or access to the 747 flight test data.
To compare the drag before and after, fly at the required altitude/speed, and notice the fuel flow with and without the gear extended.
Write down both fuel flow values, look up the jet engine model from jet-engine.net/civtfspec, note the number of engines, and use the SFC value (lb/lbf hr) to find out the force differential in lbf, then convert to Newtons.
I ran the test in a reputable 737-800 add-on, and with the values for the CFM56-7B27 engine (SFC 0.380), the difference was 67 kN at 250 knots IAS and 3000 feet MSL.
Source—CFD visualization of the turbulent structures around a landing gear.
Fuel flow figures (per engine) were 1.1 and 2.4 (x1000 kg/hr), angle of attack remained constant.
The drag (thrust) values were 57 kN and 124 kN. The landing gear more than doubled the drag at 250 knots (460 km/h).
In other words, the landing gear alone consumed 2600 kg/hr of fuel in those conditions.
That agrees with Terry's experience:
It's enough drag that in flying a 747-100 gear down the 600 nautical miles from Yokota Air Force Base near Tokyo to Osan Air Force Base a bit south of Seoul, we burnt more than twice the projected fuel burn. Had we not been tankering fuel to avoid refueling at Osan, we would not have been able to do it when we couldn't get the gear up. It was all we could do initially to stagger up to FL190, and we never got higher than FL220.—Terry
A 747 has two bogies more than a 737, they're also bigger and tilted, so the drag penalty for a 747 is likely triple that of a 737, or even more.
At approach speeds the landing gear drag should be much, much lower.
[The] drag depends linearly on the size of the object moving through the air.
[The drag varies] with the square of the relative velocity between the object and the air.