It can all be done and realized... on the right budget. To achieve this, you would need to measure or scan the part, make a 3D/CAD model (for scanning, process the data so it is usable for manufacturing components) and select the appropriate material and production method (after all, machining is not the only method to make an aluminium gearbox). If you want it anodized, that would need to be done afterwards as well.
I don't know how much you're willing to spend, but under a series of 10 units I'm not sure that you'll get them for less than 50GBP/gearbox - unless you find companies or people willing to help you for friend-prices
So it makes me curious... would you like to share why you are specifically looking to have a gearbox produced in aluminium?