How accurate of the gps timing on ambulances?
Google maps ETAs are based on a variety of things. These things range from official speed limits and recommended speeds, historical average speed data over certain time periods.
Actual travel times from previous users, and real-time traffic information. They mix data from whichever sources they have, and come up with the best prediction they can make.
A 2013 study evaluated google maps ETA vs Actual ETA for Ambulances. One thing to note is that there is a difference between Siren/Lights On vs Off. This is an older study but may still be relevant https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4288957/