Like I mentioned in the other post, the 250GT engine is a unit construction engine.
As such, the engine oil is also the gearbox oil. I don't believe there is a separate oil filler plug for the gearbox and the engine oil filler serves both the engine and the gearbox.
I will admit I may be wrong because I was reading GREAT BRITISH MOTORCYCLES by Bob Currie which has a chapter on the Continental GT 250 (pp 183-189).
In this, the author says,
"...The beauty was more than skin deep, too, because compression ratio had been raised to 9.5, the inlet port in the head ws opened out and a 1 1/8-in choke Amal Monobloc carburetor fitted- incidentally, with a bell-mouth intake instead of an air filter... Moreover, crankcase breathing had been entirely revised, the waste gasses now making their way to atmosphere by way of a big-bore plastic pipe draped along the underside of the seat...In truth, the better crankcase breathing was long overdue, because the Crusader family of engines had for years suffered from high blood pressure. There is no denying that they went like stink, but the crankcase joints were anything but oiltight, nor did it help matters that many riders tended to overfill the gearbox [engine?]. So take note, any reader thinking of restoring a little Enfield, that there is indeed a gearbox oil-level plug. It's the middle screw of the right-hand engine cover...."(p 183).
Under a cutaway drawing of the engine on page 184 the caption says, "Basic power unit of the Crusader family...The unit-construction power house has a common oil supply for the engine and gearbox."
The MOTOR CYCLE , MARCH 1966 review of the Continental GT included in the chapter shows the claimed horsepower at 21 bhp @ 7,500 rpm.
It shows the weight at 306 pounds and says the lubrication capacity is 3 pints of oil.