In Spain, in this case, we would say: "creo que me voy a marchar" "me parece que me voy a marchar". It's true guess means in general "adivinar" or "suponer", but in this very specific context, "I guess..." is and informal way to express an intention. If I said "supongo" it would be as if I don't know my own feelings and I can only "suppose" what I'm going to do, but, in fact, if nothing avoids it, I will leave. Difficult to explain. Perhaps with this example: I guess he'll leave now: supongo (imagino/creo) que se marchará (ahora); I guess I'll leave now: Me parece/creo que me voy. Anyway, this applies at least for the spanish spoken in Spain, so I cannot say that Alberto's point of view is wrong.

I guess means "supongo" o "adivino", for example: I guess I'll leave now", in Spanish is "supongo que me iré ahora".

Supongo que me iré ahora.