We know from verse 34 that Mary does understand Gabriel's message to mean she would have a miraculous conception, as you allude to in your question:
34And Mary said to the angel, “How will this be, since I am a virgin?” Luke 1, ESV
But it is not clear from the preceeding text how Mary knows that, as you say.
I think it is helpful to bear in mind here that although Luke aims "to write an orderly account", that does not mean he will necessarily aim to include every detail of every event: this is still a narrative, and the intent is that the reader would "have certainty concerning the things you have been taught" rather than a detailed timeline of events.
It is of course possible that other words were exchanged between Gabriel and Mary that he does not report, or that something about the tone or manner of his words or his expression made Mary understand his meaning, or indeed that she was simply directly given insight and understanding at the right time: but Luke chooses not to tell us because he has left no ambiguity about the facts he is concerned with communicating.
The miraculous nature of Jesus conception is in contrast with the 'less-miraculous' nature of John's conception that is interwoven in the narrative: Luke is trying very hard to tell us something about the nature of the child to be born, and not so much about Mary and what she knew and when.