It depends on where you sit. If you are in Platea or palcos you should wear a dark suit (men). Women will dress up. Before the crisis it was black tie or what the Argentines call "smoking" for men but that requirement was dropped when people supposedly complained that their opulent dress was a security risk. Anyway if you are in the cheap seats high up it makes no difference how you dress. You can't even enter through the main entrance. You have to go through a side entrance. No grand staircase, no Salon Dorado and no confiteria.
When we went to Colon (it was no gala) there were quite a few women wearing long dresses. Men without suit or at least jacket were rare. So I think you're right to check the dress code.
It wouldn't surprise me if 'gala' would really mean 'gala' (tux and long dress)... I would probably wear a nice dress and heels, my husband suit and tie. WIth this you will certainly not be 'overdressed' and you want stick out as 'underdressed' either.
There is no longer an official dress code at the Teatro Colon. As I mentioned earlier, there used to be a REQUIREMENT that men wear black tie for the Gran Abonos which I think is what you are referring to when you say "gala". In those days they told you at the box office that it was obligatory for men to wear "smoking". You will still see women in evening gowns for the Gran Abono but men will be dressed in dark suits for the most part. In the cheap seats people feel free to dress as they like. Despite the GREAT expense of the Gran Abono tickets you will still see some people dressed in casual clothes but that will be more prevalent at the other functions which are also expensive.