Do not put it in a microwave!
Just do as Nforce4max says, but be gentle and do not use any soap or detergent, just water, distilled or deionised water would be best, it can be bought at car parts or accessory shops, it's the stuff used to top-up batteries and is chemically pure so it will leave no residue.
Make sure to dry the card fully, leave it somewhere warm and dry for at least a day after you have dried it with a towel or compressed air.
Do not forget to put new thermal compound on the GPU during assembly and the old stuff is best removed using a paper towel moistened with pure alcohol, either surgical alcohol or isopropyl alcohol are widely available and leave no residue.