Hi there SK, gear boxes are usually added to a motor to reduce RPM in order to fly a larger prop. Looking at your Multiplex Minimag I see a couple problems. First, I don't think it would fit under the existing cowl,
and second you may not have the ground clearance to use a prop larger than the stock 6 x 4. But if you like experimenting here's a link to NES where they show the Permax gear box to fit your 400 http://www.nesail.com/detail.php?productID=3606 Looks like it bolts right up to the 400, the 3.3-1 gear set for example would convert 3.3 revolutions of your motor to 1 revolution of the output shaft (prop). So if you used a tachometer on the Minimag and it's turning 9900 RPM now, putting a 3.3-1 gearbox in the power train would reduce output rpm to 3000+/- requiring a much bigger prop, say 10 x 4.7 or so.

Make sense?