Word on some community forums is that Rockstar used photogrammetry to capture real Miami. An editorial on what's plausible.
What's plausible
Photogrammetry is a real technique and has been used in some open-world games. Rockstar has the resources to pull it off. Whether or not they did has yet to be confirmed.
Street-by-street photogrammetric mapping is unlikely - Rockstar creates its own.