Lugia can not learn Aeroblast in Gold. If you want it in Gold, you will need to catch it in Silver, and have it learn Aeroblast at lvl1. Otherwise, talk to the Move Reminder in Blackthorn City. If you catch it in Silver, it should know Aeroblast.
Or, in Pokemon Stadium, you can relearn one move after beating the champion, so you can have Lugia learn Aeroblast.
Hope I helped! :)