I have not tried to use Google Maps for elevation data but I have used Google Earth for elevation profiles in the past. Sometimes the data was noisy and a few times it had big errors.

For example, I looked at one route that was on a road that was next to a river at the base of a tall bluff. The river was nearly horizontal so the road elevation varied only slightly because the road was close to the shoreline. Yet sometimes Google Earth would suddenly plot the elevation at the top of the bluff instead of the bottom along the road. The error was probably lack of precision in their topographic data and/or lack of accuracy on the road location.

I used my own GPS data in Google Earth to plot an elevation profile for Going to the Sun Road, it had lots of up and down noise in the plot where the road had a nearly constant uphill grade. I suspect the problem was that Google Earth was thinking I had left the road and plotted the elevation for land surfaces adjacent to the road. But when I plotted the data in Mapsource to create an elevation profile, it showed the constant grade very well.

So, don't get your hopes too high for a perfect result in areas where the elevation of the ground surface varies quite a bit.