does anyone know how to top up the gearbox oil on a subaru impreza

On most cars, the gearbox oil is from the same resevoir as the sump oil, so just top up that.

according to the handbook you fill it through the dipstick tube
very slowly !!on left side of engine looking from the front,
page 10-19 of owners manual, 4.0 litres oil capacity,80w or 85w viscosity is recommended, good luck.
engine filler is on the right, dont mix your oils up.

If you have a WRX, you will find the dipstick hidden behind the left side (as you're looking at the engine from the front) of the intercooler..

Passenger Side of a US spec car. Look on the side of the trans from the top. Small loop is the dipstick. If its an auto it wont take much as its the front differential oil only. If its a manual it will take a long time to fill as the oil has to flow into the whole case so be paitent. Subaru transmissions are known for being finicky. Use a regular long funnel and 80/90 gl5 gear oil

On the RH sie of the car between the turbo & intercooler there is a dipstick, this is the filler for the manual transmission & front diff, fill it throught the dipstick tube using a funnel with 75w/90 gear oil till it reaches the full mark on the dipstick. they hold around 4 litres.
Autos - this dipstick is for the front diff only, fill to full make with 80w/90 gear oil.
The auto trans dipstick is on the LH side of the car, fill to full mark with normal dextron II l atf fluid

l did mine through the dip-stick, which is just under the left hand side of the intercooler.