Sorry I accidentally wrote my answer on the comment box so please ignore that one... I think generally those two are interchangeable, but すみません sounds more polite and is used in more official situations (i.e. apologizing to your boss, strangers etc.). You wouldn't say すみません to for example your parents or friends. Also, as you may know already, you can say すみません when you want to say "Excuse me." I hope this helped!

すみません　has 2 meanings. 1. Excuse me. We use when we start asking or call such as restaurants staff. 2. Sorry. We use すみません　to say sorry lightly. In business situation to apologize to customers or boss many Japanese use すみませんでした。もうしわけありません。 もうしわけありませんでした。もうしわけございません。and so on. But more important than to say a word is how we bow and voice tone. ごめんなさい　Im (so ) sorry. 　ごめんなさい is more emotional. When children say Im very sorry to their parents they are educated to say ごめんなさい。Between friends we use ごめんね　short version. When I say ごめんなさい I feel that all fault is on my side. But, in casual situation like hitting someone in the street, some polite Japanese say ごめんなさい　instead of すみません.

I think generally those two are interchangeable, but すみません sounds more polite and is used in more official situations (i.e. apologizing to your boss, strangers etc.). You wouldn't say すみません to for example your parents or friends. Also, as you may know already, you can say すみません when you want to say "Excuse me." I hope this helped!