Aluminum dissolves VERY well in vinegar, and forms highly soluble aluminum acetate, which is a very low toxicity salt. In fact, it's often used in many topical medicines for its anti-itch and astringent effects.
For this material, you would want to use ONLY vinegar with NO SALT. It will eat away the aluminum and some of the copper, but should not touch the gold at all in such a low acetate concentration. Gold(III) acetate is only commonly formed from the reaction of gold(III) hydroxide with glacial acetic acid.
The copper acetate is also low toxicity, and was used for centuries for its fungicidal effects, as well as for pigments.
So, if you can be certain this material has only aluminum, copper, and gold; then low-concentration acetic acid (vinegar) is the simplest and cheapest method to leave you with relatively clean gold foils.