you need the entity file in gamedata, the mesh it references, any abilities and/or particles (weapon fx etc) it references, sound files and the textures for the mesh. Then you need to make sure all entities are added to your manifest. Also the sound fx for the turbo lasers would have to be added to your soundfx file unless you changed it to use stock TEC sounds or something.
It only takes a couple of seconds to do, however if you have little to no experiance with modding you might want to check out some tutorials first.
I'd help you out a bit more but my main development PC is inaccessable at the moment and I don't remeber wich files exactualy the Golan III uses.
If you open the entity file you should be able to work out wich particles and such you need (from the weapon settings) and if you open the golan mesh with notepad++ or something you should be able to see what textures it uses (though i think they're named like Golan_hull or something like that anyway.)