If the orange one (the one you have) is threaded on the inside you will need a CYMA Threaded Flash Hider. If it's not threaded inside then a TM, ICS, or king Arms might work. However all of the letter need a set screw to keep them in place so you need to make sure that the Galaxy MP5K has a set screw hole to keep any of the later in place. I have no experience with the Galaxy MP5K, but did google some pictures.
http://lmgtfy.com/?q=airsoft+MP5+threaded+flash+hider