The problem with gold as a flat colour is that it just turns into a horrible yellowy-brown. Gold tends to be made up of lots of flecks of different colours.
If you can't use a gold as an additional spot colour, then I would try to create an image - add noise to a white image in photoshop, motion blur it and then add a gold coloured over lay - it won't be the same as an additional colour but may make a good substitue
What Jam says. Gold is basically a yellowly orange colour, but it's refelctive. To obtain a gold, you either need to use a Pantone Gold spot colour or simulate the way gold looks psuedo-photographically (ie, fake the reflective properties in photoshop - it won't actually be reflective but you can make it look like it is reflecting either generic light and dark sources or other elements in the design depending on what is appropriate). I'm sure there are probably onlne tutorials for doing fake gold textures in photoshop.
Using a flat CMYK will rarely, if ever, look gold.
Gold is generally smoother than brushed aluminium, it doesn't usually have so many imperfections. Really, it's a yellowy orange that reflects light and dark around it, almost matt with minimal detail, in accordance with the shape of the gold object.
I once managed to fake a gold foil wrapper for a chocolate bar quite effectively using, if I recall correctly, a combination of noise clouds, Crystalise, a yellowy orange overlay and then fashioning the outline of the blocks of chocolate on a layer that I then multiplied over the top. The yellowy orange gives the basic gold colour and then the light and dark of the crystalise and the chocolate blocks gave the impression of dimension with reflection.